Justin Trudeau was born on Christmas Day 1971 at 9:27 pm EST at the Ottawa Civic Hospital.

Trudeau was christened with his father's niece Anne Rouleau-Danis as godmother and his mother's brother-in-law Thomas Walker as godfather at Ottawa's Notre Dame Basilica on the afternoon of January 16, 1972, which marked his first public appearance.

On April 14, 1972, Trudeau's father and mother hosted a gala at the National Arts Centre, at which visiting U.S. president Richard M. Nixon said, "I'd like to toast the future prime minister of Canada, to Justin Pierre Trudeau" to which Pierre Elliott Trudeau responded that should his son ever assume the role, he hoped he would have "the grace and skill of the president".

His parents publicly announced their separation on May 27, 1977, when Trudeau was five years old, with his father having custody.

Trudeau lived at 24 Sussex Drive, Ottawa, the official residence of Canada's prime minister, from his birth until his father's government was defeated in the federal election on May 22, 1979.

They reconnected as adults in June 2003, when Grégoire, by then a Quebec television personality, was assigned as Trudeau's co-host for a charity ball; they began dating several months later. Trudeau and Grégoire became engaged in October 2004, and married on May 28, 2005, in a Roman Catholic ceremony at Montreal's Sainte-Madeleine d'Outremont Church.

On September 17, 2006, Trudeau was the master of ceremonies at a Toronto rally organized by Roméo Dallaire that called for Canadian participation in resolving the Darfur crisis.

In October 2006, Trudeau criticized Quebec nationalism by describing political nationalism generally as an "old idea from the 19th century", "based on a smallness of thought" and not relevant to modern Quebec.

In 2007, Trudeau starred in the two-part CBC Television miniseries The Great War, which gave an account of Canada's participation in the First World War.

Trudeau faced off against Mary Deros, a Montreal city councilor and Basilio Giordano, the publisher of a local Italian-language newspaper for the Liberal nomination. On April 29, 2007, he easily won the party's nomination, picking up 690 votes to 350 for Deros and 220 for Giordano.

Prime Minister Stephen Harper called an election for 14 October 2008, by which time Trudeau had been campaigning for a year in Papineau. On election day Trudeau narrowly defeated Bloc Québécois incumbent Vivian Barbot.

In September 2010, he was reassigned as a critic for youth, citizenship, and immigration. During that time, he criticized the government's legislation targeting human smuggling, which he argued would penalize the victims of smuggling.

Trudeau was re-elected in Papineau in the 2011 federal election, as the Liberal Party fell to third-party standing in the House of Commons with only thirty-four seats.

On September 26, 2012, multiple media outlets started reporting that Trudeau would launch his leadership bid the following week.

On October 2, 2012, Trudeau held a rally in Montreal to launch his bid for the leadership of the Liberal Party.

The leadership campaign three by-elections were held on November 26, 2012.

Trudeau was declared the winner of the leadership election on April 14, 2013, garnering 80.1% of 30,800 votes.

On January 27, 2014, Trudeau and MP Carolyn Bennett escorted Chrystia Freeland into the House of Commons, as is traditional for by-election victors.

On October 19, 2015, after the longest official campaign in over a century, Trudeau led the Liberals to a decisive victory in the federal election. The Liberals won 184 of the 338 seats, with 39.5% of the popular vote, for a strong majority government; a gain of 150 seats compared to the 2011 federal election.

On October 22, 2015, Trudeau stated that, once prime minister, he would end Canada's airstrike mission against ISIS.

Trudeau and the rest of the Cabinet were sworn in by Governor General David Johnston on November 4, 2015.

On November 5, 2015, during the first Liberal caucus meeting since forming a majority government, the party announced that it would reinstate the mandatory long-form census that had been scrapped in 2010, effective with the 2016 census.

He said that his first legislative priority was to lower taxes for middle-income Canadians and raise taxes for the top one percent of income earners after parliament was reconvened on December 3, 2015.

On June 14, 2016, Trudeau refused to recognize ISIS's atrocities as genocide, instead waiting for an official position from the UN Security Council. He switched his position once the UN Commission released its inquiry on Syria.

The legislation to legalize cannabis for recreational use (Cannabis Act, Bill C-45) was passed by the House of Commons of Canada in late November 2017; it passed second reading in the Senate of Canada on March 22, 2018.

On June 18, 2018, the House passed the bill with most, but not all, of the Senate's amendments.

As of October 17, 2018, Cannabis is legal in Canada for both recreational and medicinal use.

Prime Minister Justin Trudeau announced the next day that recreational use of cannabis would no longer violate criminal law as of October 17, 2018.