Patrick was a 5th-century Romano-British Christian missionary and bishop in Ireland. Much of what is known about Saint Patrick comes from the Declaration, which was allegedly written by Patrick himself. It is believed that he was born in Roman Britain in the fourth century, into a wealthy Romano-British family. His father was a deacon and his grandfather was a priest in the Christian church. According to the Declaration, at the age of sixteen, he was kidnapped by Irish raiders and taken as a slave to Gaelic Ireland. It says that he spent six years there working as a shepherd and that during this time he "found God". The Declaration says that God told Patrick to flee to the coast, where a ship would be waiting to take him home. After making his way home, Patrick went on to become a priest.

Tradition holds that he died on 17 March and was buried at Downpatrick. Over the following centuries, many legends grew up around Patrick and he became Ireland's foremost saint.

Saint Patrick's feast day, as a kind of national day, was already being celebrated by the Irish in Europe in the ninth and tenth centuries. In later times, he became more and more widely seen as the patron of Ireland. Saint Patrick's feast day was finally placed on the universal liturgical calendar in the Catholic Church due to the influence of Waterford-born Franciscan scholar Luke Wadding in the early 1600s. Saint Patrick's Day thus became a holy day of obligation for Roman Catholics in Ireland. It is also a feast day in the Church of Ireland, which is part of the worldwide Anglican Communion.

St Patrick's Day, while not a legal holiday in the United States, is nonetheless widely recognized and observed throughout the country as a celebration of Irish and Irish-American culture. Celebrations include prominent displays of the color green, religious observances, numerous parades, and copious consumption of alcohol. The holiday has been celebrated in what is now the U.S since 1601.

On St Patrick's Day, it is customary to wear shamrocks, green clothing or green accessories. St Patrick is said to have used the shamrock, a three-leaved plant, to explain the Holy Trinity to the pagan Irish. This story first appears in writing in 1726, though it may be older. In pagan Ireland, three was a significant number and the Irish had many triple deities, a fact that may have aided St Patrick in his evangelization efforts. Patricia Monaghan says there is no evidence that the shamrock was sacred to the pagan Irish. However, Jack Santino speculates that it may have represented the regenerative powers of nature, and was recast in a Christian context‍—‌icons of St Patrick often depict the saint "with a cross in one hand and a sprig of shamrocks in the other". Roger Homan writes, "We can perhaps see St Patrick drawing upon the visual concept of the triskele when he uses the shamrock to explain the Trinity".

The island of Montserrat is known as the "Emerald Island of the Caribbean" because of its founding by Irish refugees from Saint Kitts and Nevis. Montserrat is one of three places where St Patrick's Day is a public holiday, along with Ireland and the Canadian province of Newfoundland & Labrador. The holiday in Montserrat also commemorates a failed slave uprising that occurred on 17 March 1768.

One of the longest-running and largest St Patrick's Day parades in North America occurs each year in Montreal, whose city flag includes a shamrock in its lower-right quadrant. The yearly celebration has been organized by the United Irish Societies of Montreal since 1929. The parade has been held yearly without interruption since 1824. St Patrick's Day itself, however, has been celebrated in Montreal since as far back as 1759 by Irish soldiers in the Montreal Garrison following the British conquest of New France.

The first Saint Patrick's Day celebrations in Malta took place in the early 20th century by soldiers of the Royal Dublin Fusiliers who were stationed in Floriana. Celebrations were held in the Balzunetta area of the town, which contained a number of bars and was located close to the barracks. The Irish diaspora in Malta continued to celebrate the feast annually.

In England, the British Royals traditionally present bowls of shamrock to members of the Irish Guards, a regiment in the British Army, following Queen Alexandra introducing the tradition in 1901.

The first St Patrick's Day parade in Ireland was held in Waterford in 1903. The week of St Patrick's Day 1903 had been declared Irish Language Week by the Gaelic League and in Waterford, they opted to have a procession on Sunday 15 March. The procession comprised the Mayor and members of Waterford Corporation, the Trades Hall, the various trade unions and bands who included the 'Barrack St Band' and the 'Thomas Francis Meagher Band'. The parade began at the premises of the Gaelic League in George's St and finished in the Peoples Park, where the public was addressed by the Mayor and other dignitaries. On Tuesday 17 March, most Waterford businesses—including public houses—were closed and marching bands paraded as they had two days previously. The Waterford Trades Hall had been emphatic that the National Holiday be observed.

In 1903, St Patrick's Day became an official public holiday in Ireland. This was thanks to the Bank Holiday (Ireland) Act 1903, an act of the United Kingdom Parliament introduced by Irish Member of Parliament James O'Mara. O'Mara later introduced the law which required that public houses be shut on 17 March after drinking got out of hand, a provision that was repealed in the 1970s.

On St Patrick's Day 1916, the Irish Volunteers—an Irish nationalist paramilitary organization—held parades throughout Ireland. The authorities recorded 38 St Patrick's Day parades, involving 6,000 marchers, almost half of whom were said to be armed. The following month, the Irish Volunteers launched the Easter Rising against British rule. This marked the beginning of the Irish revolutionary period and led to the Irish War of Independence and the Civil War. During this time, St Patrick's Day celebrations in Ireland were muted, although the day was sometimes chosen to hold large political rallies. The celebrations remained low-key after the creation of the Irish Free State; the only state-organized observance was a military procession and trooping of the colors, and an Irish-language mass attended by government ministers.

In Malaysia, the St Patrick's Society of Selangor, founded in 1925, organizes a yearly St Patrick's Ball, described as the biggest St Patrick's Day celebration in Asia. Guinness Anchor Berhad also organizes 36 parties across the country in places like the Klang Valley, Penang, Johor Bahru, Malacca, Ipoh, Kuantan, Kota Kinabalu, Miri, and Kuching.

In 1927, the Irish Free State government banned the selling of alcohol on St Patrick's Day, although it remained legal in Northern Ireland. The ban was not repealed until 1961.

The first official, state-sponsored St Patrick's Day parade in Dublin took place in 1931.

The church calendar avoids the observance of saints' feasts during certain solemnities, moving the saint's day to a time outside those periods. St Patrick's Day is occasionally affected by this requirement, when 17 March falls during Holy Week. This happened in 1940 when Saint Patrick's Day was observed on 3 April to avoid it coinciding with Palm Sunday, and again in 2008, where it was officially observed on 15 March. St Patrick's Day will not fall within Holy Week again until 2160. However, the popular festivities may still be held on 17 March or on a weekend near to the feast day.

Irish Government Ministers travel abroad on official visits to various countries around the globe to celebrate St Patrick's Day and promote Ireland. The most prominent of these is the visit of the Irish Taoiseach (Irish Prime Minister) with the U.S. President which happens on or around St Patrick's Day. Traditionally the Taoiseach presents the U.S. President a Waterford Crystal bowl filled with shamrocks. This tradition began when in 1952, Irish Ambassador to the U.S. John Hearne sent a box of shamrocks to President Harry S. Truman. From then on it became an annual tradition of the Irish ambassador to the U.S. to present the St Patrick's Day shamrock to an official in the U.S. President's administration, although on some occasions the shamrock presentation was made by the Irish Taoiseach or Irish President to the U.S. President personally in Washington, such as when President Dwight D. Eisenhower met Taoiseach John A. Costello in 1956 and President Seán T. O'Kelly in 1959 or when President Ronald Reagan met Taoiseach Garret FitzGerald in 1986 and Taoiseach Charles J. Haughey in 1987. However, it was only after the meeting between Taoiseach Albert Reynolds and President Bill Clinton in 1994 that the presenting of the shamrock ceremony became an annual event for the leaders of both countries for St Patrick's Day.

The Irish Association of Korea has celebrated Saint Patrick's Day since 1976 in Seoul, the capital city of South Korea. The place of the parade and festival has been moved from Itaewon and Daehangno to Cheonggyecheon.

In Northern Ireland, the celebration of St Patrick's Day was affected by sectarian divisions. A majority of the population were Protestant Ulster unionists who saw themselves as British, while a substantial minority were Catholic Irish nationalists who saw themselves as Irish. Although it was a public holiday, Northern Ireland's unionist government did not officially observe St Patrick's Day. During the conflict known as the Troubles (the late 1960s–late 1990s), public St Patrick's Day celebrations were rare and tended to be associated with the Catholic community. In 1976, loyalists detonated a car bomb outside a pub crowded with Catholics celebrating St Patrick's Day in Dungannon; four civilians were killed and many injured. However, some Protestant unionists attempted to 're-claim' the festival, and in 1985 the Orange Order held its own St Patrick's Day parade. Since the end of the conflict in 1998, there have been cross-community St Patrick's Day parades in towns throughout Northern Ireland, which have attracted thousands of spectators.

St Patrick's parades are now held in many locations across Japan. The first parade, in Tokyo, was organized by The Irish Network Japan (INJ) in 1992.

The first St Patrick's Day parade in Russia took place in 1992.

The first St Patrick's Festival was held on 17 March 1996. In 1997, it became a three-day event, and by 2000 it was a four-day event. By 2006, the festival was five days long; more than 675,000 people attended the 2009 parade. Overall 2009's five-day festival saw almost 1 million visitors, who took part in festivities that included concerts, outdoor theatre performances, and fireworks. The Skyfest which ran from 2006 to 2012 formed the centerpiece of the St Patrick's festival.

Since 1999, there has been a yearly "Saint Patrick's Day" festival in Moscow and other Russian cities. The official part of the Moscow parade is a military-style parade and is held in collaboration with the Moscow government and the Irish embassy in Moscow. The unofficial parade is held by volunteers and resembles a carnival.

On two occasions, parades across the Republic of Ireland have been canceled from taking place on St Patrick´s Day, with both years involving health and safety reasons. In 2001, as a precaution to the foot-and-mouth outbreak, St Patrick´s Day celebrations were postponed to May

London, since 2002, has had an annual St Patrick's Day parade which takes place on weekends around the 17th, usually in Trafalgar Square. In 2008 the water in the Trafalgar Square fountains was dyed green.

In Buenos Aires, a party is held in the downtown street of Reconquista, where there are several Irish pubs; in 2006, there were 50,000 people in this street and the pubs nearby. Neither the Catholic Church nor the Irish community, the fifth largest in the world outside Ireland, takes part in the organization of the parties.

Glasgow has a considerably large Irish population; due, for the most part, to the Irish immigration during the 19th century. This immigration was the main cause of raising the population of Glasgow by over 100,000 people. Due to this large Irish population, there are many Irish-themed pubs and Irish interest groups who hold yearly celebrations on St Patrick's day in Glasgow. Glasgow has held a yearly St Patrick's Day parade and festival since 2007.

In 2014, Moscow Irish Week was celebrated from 12 to 23 March, which includes St Patrick's Day on 17 March. Over 70 events celebrating Irish culture in Moscow, St Petersburg, Yekaterinburg, Voronezh, and Volgograd were sponsored by the Irish Embassy, the Moscow City Government, and other organizations.

Sarajevo, the capital city of Bosnia and Herzegovina has a large Irish expatriate community. The community established the Sarajevo Irish Festival in 2015, which is held for three days around and including St. Patrick's Day. The festival organizes an annual parade, hosts Irish theatre companies, screens Irish films and organizes concerts of Irish folk musicians. The festival has hosted numerous Irish artists, filmmakers, theatre directors and musicians such as Conor Horgan, Ailis Ni Riain, Dermot Dunne, Mick Moloney, Chloë Agnew, and others.

In 2017, the Russian Orthodox Church added the feast day of Saint Patrick to its liturgical calendar, to be celebrated on 30 March [O.S. 17 March].

In 2020, as a consequence of the severity of the coronavirus (COVID-19) outbreak, the St Patrick´s Day Parade was canceled outright.