Aaron Josef Hernandez was born in Bristol, Connecticut, and raised on Greystone Avenue. He was the son of Dennis Hernandez, of Puerto Rican descent, and Terri Valentine-Hernandez, of Italian descent. As an adult, Hernandez remembered his mother throwing his father out of the house on multiple occasions, but always letting him back in. The couple married in 1986, divorced in 1991, and remarried in 1996. In 1991, they filed for bankruptcy. Hernandez would later state there was constant fighting going on in the home. Both parents would be arrested and involved in crime during their lives.

The beatings Hernandez's father gave him and his brother were sometimes for no reason at all or were alcohol-related, but often came when their father believed they were not trying hard enough in school or athletics. D.J. and Hernandez lived in constant fear of their father but also revered him. Hernandez once came to school with a black eye that his coach believed resulted from his father hitting him. His father once punched Hernandez's youth football coach after a dispute about coaching methods. According to Hernandez's brother D.J., Hernandez was also sexually molested as a child. A teenage boy in his babysitter's house forced Hernandez to perform oral sex on him beginning when Hernandez was six years old and continuing for several years. A college girlfriend said that "he never dealt with the sexual abuse. It led to issues in his sexuality."

Hernandez attended Bristol Central High School, where he played for the Bristol Rams football team. He was also an exceptional basketball player and track runner. He started as a wide receiver before becoming a tight end, and also played defensive end. As a senior, he was Connecticut's Gatorade Football Player of the Year after making 67 receptions for 1,807 yards and 24 touchdowns on offense and 72 tackles, twelve sacks, three forced fumbles, two fumble recoveries, and four blocked kicks on defense. He was also a US Army All-American.

Hernandez's father died in January 2006, from complications from hernia surgery, when Hernandez was 16. According to his mother, Hernandez was greatly affected by his father's death, and he acted out his grief by rebelling against authority figures. Those who knew him said he never got over his father's death. Hernandez became estranged from his mother and largely moved in with Tanya Singleton, his older cousin. Following Dennis's death, the family learned that Terri Hernandez and Singleton's husband, Jeff Cummings, had been having an extramarital affair. After the affair became public, Singleton and Cummings divorced, and Cummings moved in with Terri. This "enraged" Hernandez. It was while he was living with Singleton that Hernandez became more involved in criminal activity.

At first, Hernandez committed to play at the University of Connecticut with his brother D.J. but ultimately chose to play for the University of Florida under head coach Urban Meyer. Meyer flew to Connecticut and convinced Hernandez's principal to allow him to graduate more than a semester early. This allowed Hernandez to move to Florida, join the team, and learn the playbook shortly after his 17th birthday. The Boston Globe later opined that

In 2007, Hernandez began dating Shayanna Jenkins, a girl he had known since elementary school. Their daughter was born in 2012, the same month that the couple became engaged and that Hernandez purchased a $1.3 million, 8,130 square foot (755 m2), four-story home with an in-ground pool in North Attleborough, Massachusetts, where the family lived together. Jenkins moved in with Hernandez in 2011, during his second season with the Patriots. After she discovered him cheating on her, Jenkins moved out but returned in the summer of 2012. During Hernandez's trial for the murder of Odin Lloyd, it was claimed that Hernandez had flirted with and kissed the nanny who took care of his daughter. Jenkins testified in court that she wanted to make their relationship work and that it required her to compromise on some of his behavior. She told police that she cooked and cleaned and she knew her role. Hernandez and Jenkins never married.

Between practices, games, team meetings, and other events, Hernandez put 40 to 60 hours a week into football, nearly year-round. He later said that he was high on drugs every time he took the field. As a freshman in 2007, Hernandez started three games for the Florida Gators. He finished the season with nine receptions for 151 yards and two touchdowns. Though he excelled in his freshman year, he was benched in the season opener of his sophomore year due to a failed drug test. Following that, he started eleven of thirteen games during the 2008 season in place of the injured Cornelius Ingram and finished the season with 34 receptions for 381 yards and five touchdowns. In the 2009 BCS National Championship Game against the Oklahoma Sooners, Hernandez led the Gators in receiving yards with 57 on five receptions, as the Gators defeated the Sooners 24–14 to win their second BCS championship in three seasons.

On April 28, 2007, according to a police report in Gainesville, Florida, 17-year-old Hernandez consumed two alcoholic drinks in a restaurant with Tim Tebow, refused to pay the bill, and was escorted out by a restaurant employee. As the manager walked away, Hernandez "sucker punched" him on the side of the head, rupturing his eardrum. The police responded at 1:17 a.m. Hernandez called Coach Urban Meyer, and Meyer called Huntley Johnson, the team's unofficial defense lawyer. The victim later told police that he had been contacted by lawyers and the team and that a settlement was being worked out, something the team denied. The police department recommended charging Hernandez with a felony battery, but the incident was settled out of court with a deferred prosecution agreement.

On September 30, 2007, someone approached a car containing Randall Carson, Justin Glass, and Corey Smith on foot and fired five shots while they were waiting at a Gainesville traffic light after having left a nightclub. Smith was shot in the back of the head, and Glass was shot in the arm. Both men survived. Carson, a back-seat passenger, was uninjured and told police that the shooter was a "Hawaiian" or "Hispanic" male with a large build weighing about 230 lb (100 kg) and having many tattoos. He picked a photo of Hernandez out of a police lineup.

Meyer had wanted to throw Hernandez off the team for his chronic marijuana use but relented after an appeal from Tebow. However, after Hernandez's junior year Meyer told him that he would not be welcome back for the fourth year and that he would have to try to get picked up by a professional team in the 2010 NFL Draft. Hernandez finished his college career with 111 receptions for 1,382 yards and twelve touchdowns. Florida coaches aligned Hernandez with Maurkice and Mike Pouncey. He reportedly grew close with the twins after rooming with them and staff considered the Pouncey's a positive influence on Hernandez.

On January 6, 2010, Hernandez announced his decision to forgo his remaining eligibility and enter the 2010 NFL Draft. He attended the NFL Scouting Combine but was unable to perform any physical drills after tearing a muscle in his back during the offseason. On March 17, 2010, Hernandez participated at Florida's pro day and performed all of the combine drills. His time in the 40-yard dash would have ranked fourth among all tight ends at the NFL Combine. Hernandez also performed 30 repetitions of 225 pounds on the bench press and would have been the top performance of all tight ends, surpassing Dennis Pitta's top performance of 27 repetitions.

The New England Patriots selected Hernandez in the fourth round (113th overall) of the 2010 NFL Draft. The previous day, the Patriots drafted Arizona tight end Rob Gronkowski. Hernandez was the sixth tight end drafted in 2010. Despite his being considered a top tight end prospect, multiple teams reportedly chose not to draft him because "he was a problem."

On June 8, 2010, the Patriots signed Hernandez to a four-year, $2.37 million contract that included a signing bonus of $200,000. The terms of his contract limited Hernandez's signing bonus to $200,000, which was less than half the signing bonus received by Patriots' fourth-round pick (118th overall) placekicker Stephen Gostkowski in 2006. The Patriots declined to give Hernandez the expected $500,000 signing bonus as a precautionary measure. To compensate for the smaller signing bonus, he received a contract that included a series of roster and workout bonuses up to an additional $700,000. If Hernandez reached all bonuses and escalators he could receive an annual salary comparable to a third-rounder, but would have to "walk the straight and narrow line to do so".

Hernandez made his professional regular-season debut and the first career start in the Patriots' season-opener against the Bengals and recorded one reception for 45-yards during their 38–24 victory. On September 19, 2010, he caught a season-high six receptions for a total of 101 receiving yards during the Patriots' 28–14 loss at the New York Jets in Week 2. Hernandez became the youngest player since 1960 to have more than 100 receiving yards in a single game. In Week 3, Hernandez led all Patriots receivers with six catches for 65-yards during a 38–30 win against the Buffalo Bills. He also had his first career carry for a three-yard gain against the Bills in Week 3. On November 7, 2010, Hernandez caught five passes for 48-yards and scored the first two touchdowns of his career during a 34–14 loss at the Cleveland Browns in Week 9. He caught his first career touchdown on a two-yard pass by Patriots' quarterback Tom Brady during the second quarter. His second touchdown of the game was scored on a one-yard pass by Brady in the fourth quarter.

The Patriots finished the 2010 season first in the AFC East with a 14–2 record and earned a first-round bye. On January 16, 2011, Hernandez started in his first career playoff game and caught one pass for a four-yard gain as the Patriots lost 28–21 against the New York Jets in the AFC Divisional Round.

At 3:45 a.m. on April 30, 2011, police responded to a fight in front of Hernandez's rented townhouse in Plainville, Massachusetts. A high school friend had been pulled over earlier in the evening after driving Hernandez home from a Boston bar. The driver was weaving in and out of lanes and traveling at 120 miles per hour in a work zone and on a highway with a speed limit of 55 miles per hour. The Massachusetts State Trooper who pulled the car over did not arrest the driver because he recognized Hernandez in the passenger seat. The Plainville police also recognized Hernandez, and told the two to go indoors.

In 2012, Hernandez told his agent that he got his respect through weapons. After his death, his high school teammate and lover said that being drafted by the Patriots "was the worst thing the NFL could have done" because it put him back into close proximity to the criminal friends he had in Connecticut. As a Patriot, Hernandez hired two of his friends from Bristol, both of whom had criminal records, as assistants. One of them, Alexander S. Bradley, was his drug dealer. As Hernandez's assistant, Bradley's other duties including calming Hernandez down during fits of rage and paranoia, and obtaining weapons for him.

Hernandez was investigated in connection with a double homicide that took place on July 16, 2012, near the Cure Lounge in Boston's South End. Daniel Jorge Correia de Abreu, 29, and Safiro Teixeira Furtado, 28, both immigrants from Cape Verde and living in Dorchester, were killed by gunshots fired into their vehicle. Witnesses testified that Hernandez's silver SUV pulled up next to the victims and someone from his car yelled, "What's up now, niggers?" Someone from the car then fired five shots, killing the two immigrants. Police immediately identified Hernandez, who was then playing for the Patriots, in the club's security camera footage, but thought it was a coincidence that the NFL star happened to be at the club that evening.

On August 27, 2012, the Patriots signed Hernandez to a five-year, $39.58 million contract extension that included $15.95 million guaranteed and a signing bonus of $12.50 million. The $12.5 million signing bonus was the largest signing bonus ever received by an NFL tight end. His $40 million total was the second-largest contract extension ever given to a tight end, after teammate Gronkowski's $53 million. Hernandez gave $50,000 of that bonus to a charity named for the late wife of the Patriots owner. Hernandez was sidelined during the Patriots' Week 2 game against the Arizona Cardinals with a high ankle sprain and missed several weeks. On December 10, during the Monday Night Football game against the Houston Texans, Hernandez recorded eight receptions for 58 yards and two touchdowns. Hernandez's last NFL appearance was the 2012 AFC Championship game on January 20, 2013, against the Baltimore Ravens.

In January 2013, Hernandez and Bradley partied at Cure again. At 2:20 a.m., Bradley was pulled over on the Southeast Expressway doing 105 miles per hour. According to the State Police, he was "wobbly drunk." Hernandez tried to get his friend out of trouble by saying, "Trooper, I am Aaron Hernandez. It's okay." However, Bradley was still arrested for drunk driving.

Hernandez traveled to California with Jenkins and their young daughter in 2013 to have shoulder surgery. While there, Jenkins called the police twice in less than a week, claiming that Hernandez was drunk and violent. In the first incident, Hernandez put his hand through a window. Hernandez's brother and friends later said that there were drugs and guns in the rented apartment, but police determined that Jenkins and the child were not in danger and never searched the premises. D.J. found Hernandez alone on the roof of the building one night, looking defeated and rubbing the barrel of a gun against his face.

In February 2013, Hernandez, Bradley, and several others visited a Florida strip club where they rang up a $10,000 bill. Hernandez began to worry about two men sitting across from them, thinking they were plainclothes Boston police officers. Bradley later recalled telling Hernandez that they were probably tracking the pair as part of their investigation into the double murder outside the Cure. Hernandez and Bradley had a troubled relationship at this point. Bradley claimed that on February 13, 2013, during the same trip, he woke up in a car with Hernandez pointing a gun at his face. The next morning police found Bradley lying in a parking lot and bleeding from a bullet hole between his eyes. Bradley survived but lost his right eye. He did not cooperate with the police, but instead sought revenge. The pair would trade more than 500 text messages in the next three months, which included death threats and attempts at extortion. Bradley told Hernandez that he had "semiautomatic weapons, bulletproof vests, and a crew that ran six deep." Hernandez's agent tried, unsuccessfully, to settle the matter quietly. Bradley demanded $5 million to keep his silence, and Hernandez countered with $1.5 million. Bradley then asked for $2.5 million. Hernandez did not respond but instead went to see his lawyer.

On June 13, 2013, Bradley filed a civil lawsuit for damages against Hernandez in a Florida federal court. He withdrew the suit four days later, giving the two a chance to work out a settlement without the media knowing about it. On September 3, 2013, Hernandez's lawyers filed a postponement request in federal court until his murder charges were resolved. In February 2016, Hernandez reached a settlement with Bradley over the lawsuit. The terms of the settlement were not disclosed.

On June 18, 2013, police searched Hernandez's home in connection with an investigation into the shooting death of a friend, Odin Lloyd, whose body was found, with multiple gunshot wounds to the back and chest, in an industrial park about a mile from Hernandez's house. The following day, Hernandez assured Patriots head coach Bill Belichick and owner Robert Kraft that he had nothing to do with the shooting. Despite this, Hernandez was "barred" from Gillette Stadium lest it becomes "the site of a media stakeout." The team also decided, a week before his eventual arrest, to cut ties with Hernandez if he was arrested on any charge related to the case.

On June 26, 2013, Hernandez was charged with first-degree murder, in addition to five gun-related charges. The Patriots released Hernandez from the team about ninety minutes later, before officially learning the charges against him. Two other men were also arrested in connection with Lloyd's death. On August 22, 2013, Hernandez was indicted by a grand jury for the murder of Lloyd; he pled not guilty on September 6, 2013. On April 15, 2015, he was found guilty of murder in the first degree, a charge that in Massachusetts automatically carries a sentence of life in prison without a possibility of parole; he also was found guilty of five firearm charges. A motive for the murder was never definitively established. Police investigated the possibility that Lloyd may have learned of Hernandez's bisexuality and that Hernandez was worried that Lloyd might out him to others.

On May 15, 2014, Hernandez was indicted on murder charges for the killings of de Abreu and Furtado, with additional charges of armed assault and attempted murder associated with shots fired at the surviving occupants in the vehicle. The trial began on March 1, 2017. The prosecution case was strongly based on testimony by Bradley, a known drug dealer who had been feuding with Hernandez since the NFL player allegedly shot him in the face and left him to die. Hernandez and Bradley each claimed the other person pulled the trigger. Jose Baez, Hernandez's attorney, argued that the proposed motive was implausible, and Hernandez was a suspect of convenience to close two unsolved murders. Bradley alleged that Hernandez was angered after the victims spilled a drink on him at a nightclub several hours before the shooting and killed them in retaliation. Security camera footage confirmed Hernandez was in the club for less than ten minutes. In that time he calmly posed for a photo with a fan and left by himself — contradicting Bradley's testimony that he departed with Hernandez. Furthermore, Baez characterized the police investigation as extraordinarily sloppy (e.g., the victims' bodies were kept in their bullet-riddled vehicle as it was towed away from the shooting scene, a major protocol violation) with no physical evidence tying Hernandez to the murders.

On May 11, 2015, Hernandez was indicted for witness intimidation in relation to the Bradley shooting, since Bradley was reportedly a witness to the 2012 Boston double homicide. The intimidation charge for Hernandez carried a maximum penalty of ten years in prison. This charge was included in Hernandez's trial for the double homicide, which began on March 1, 2017. During the trial, it was revealed Bradley texted his lawyer this about the shooting in a deleted text message: "Now u sure once I withdraw this lawsuit I won't be held on perjury after I tell the truth about me not recalling anything about who shot me."

State Department of Correction spokesman Christopher Fallon first said that no suicide note was found in the initial search of the two-person cell, which Hernandez occupied alone. Shampoo was found covering the floor, cardboard was wedged under the cell door to make it difficult for someone to enter, and there were drawings in blood on the walls showing an unfinished pyramid and the all-seeing eye of God, with the word "Illuminati" written in capital letters underneath. On April 20, 2017, investigators reported that three handwritten notes were next to a Bible opened to John 3:16 and that "John 3:16" was written on his forehead in red ink.

Hernandez was later acquitted of the charge of witness intimidation by a jury on April 14, 2017. They also acquitted Hernandez of all other charges in the murders of de Abreu and Furtado, except for finding him guilty on one count of illegal possession of firearms.

On April 19, 2017, at 3:05 a.m. EDT, five days after Hernandez was acquitted of the 2012 Boston double homicide of Daniel de Abreu and Safiro Furtado, correction officers found Hernandez hanging by his bedsheets from the window in his cell at the Souza-Baranowski Correctional Center in Lancaster, Massachusetts. He was transported to UMass Memorial Hospital-Leominster, where he was pronounced dead at 4:07 am. He had been smoking K2, a drug associated with psychosis, within thirty hours of his death.

After Hernandez's death, on April 25, 2017, his lawyers filed a motion at Massachusetts Superior Court in Fall River to vacate his murder conviction. The request was granted on May 9, 2017; therefore Hernandez technically died an innocent man, due to the legal principle of abatement ab initio. Under Massachusetts law, this principle asserts that when a criminal defendant dies but has not exhausted all legal appeals, the case reverts to its status "at the beginning"—the conviction is vacated and the defendant is rendered "innocent." At the time of his death, Hernandez was in the process of filing an appeal for his 2015 conviction in the murder of Odin Lloyd. As of May 9, 2017, the date of the judge's ruling to vacate, the Bristol County district attorneys stated they planned to appeal the ruling, to the Massachusetts Supreme Court if necessary. The Lloyd family was disappointed with the ruling, but their attorney did not believe it would affect the wrongful death civil suit which the family had filed.

The appeal was heard by the Massachusetts Supreme Judicial Court in November 2018, a year after Hernandez's death, by six justices. The attorney representing the Lloyd family, Thomas M. Quinn, III, argued that Hernandez was rightfully convicted of Lloyd's murder and that the conviction was unfairly wiped out. Quinn also argued that Hernandez killed himself knowing of the technicality that would get his conviction thrown out, and that, "He should not be able to accomplish in death, what he never would have been able to do in life." On March 13, 2019, the Supreme Judicial Court reinstated Hernandez's conviction but stated that the trial record would note that his conviction was "neither affirmed nor reversed"; the appeal was rendered moot because Hernandez died while the case was on appeal. The Court, in their ruling, also officially ended the practice of abatement ab initio, ruling that it was outdated, never made sense, and that it was "no longer consonant with the circumstances of contemporary life, if, in fact, it ever was." After the ruling, Hernandez's estate vowed to appeal the ruling further.

A Netflix documentary entitled Killer Inside: The Mind of Aaron Hernandez was released for streaming on January 15, 2020.