The 43-year old former French teacher overcame campaign attack ads claiming he was "just not ready," winning the October 19 election by a landslide and bringing an end to nine years of Tory rule.
With his dark mop of curly hair, a confident swagger and hints of his father's speech, the younger Trudeau stood tall, smiled and mouthed "Thank you" as applause erupted after he took the oath in the ballroom of the governor general's mansion.
Earlier, Trudeau's mother Margaret had been first to arrive for her son's swearing-in at Rideau Hall -- the official residence of the governor general -- carrying his youngest son Hadrian and with his two other children, Ella-Grace and Xavier, in tow.
Trudeau himself and his cabinet pulled up soon afterward in a bus, eschewing individual limousines traditionally used to ferry government ministers around the capital, and then walked up a winding lane to the governor's mansion.
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His team includes some well-known personalities such as former astronaut Marc Garneau, former Liberal leader Stephane Dion, as well as several fresh faces.
For the first time ever, the Canadian public was officially invited to witness the swearing-in ceremony in person. Two massive screens were set up outside the mansion for additional public viewing.
Many people had travelled to Ottawa from Montreal and Toronto for the ceremony, and camped out to get a glimpse of the new prime minister, who glad-handed dozens of people as he walked in with his wife Sophie Gregoire and his inner circle.
The family is often compared to the Kennedys in the United States.
At 43, Trudeau is the nation's second youngest prime minister after Joe Clark in 1979.
Trudeau was first elected to the House of Commons in 2008, after years trying out different roles. He worked as a whitewater guide, snowboard instructor, bartender, bouncer and public speaker.
After the ballroom ceremony, he was expected to deliver a statement and hold the new government's first cabinet meeting.