The Liberals seized a Parliamentary majority, an unprecedented turn in political fortunes that smashed the record for the number of seats gained from one election to the next. The Liberals had been a distant third place party in Parliament before this election.
Harper conceded defeat, ending his government's nine-year run in power and the 56-year-old's brand of fiscal and cultural conservatism.
Trudeau, 43, the photogenic son of former Prime Minister Pierre Trudeau, pledged to run a C$10 billion annual budget deficit for three years to invest in infrastructure and help stimulate Canada's anemic economic growth.
This rattled financial markets ahead of the vote and the Canadian dollar weakened on news of his victory.
Trudeau has said he will repair Canada's cool relations with the Obama administration, withdraw Canada from the combat mission against Islamic State militants in favor of humanitarian aid and training, and tackle climate change.
Trudeau vaulted from third place to lead the polls in the final days of the campaign, overcoming Conservative attacks that he is too inexperienced to govern to return to the Prime Minister's residence in Ottawa where he grew up as a child.
"When the time for change strikes, it's lethal," former Conservative Prime Minister Brian Mulroney said in a television interview. "I ran and was successful because I wasn't Pierre Trudeau. Justin is successful because he isn't Stephen Harper."
The Conservatives were projected to become the official opposition in Parliament, with the left-leaning New Democratic Party in third.
Liberal supporters at the party's campaign headquarters broke into cheers and whistles when television projected that Trudeau would be the next prime minister.
Top Trudeau advisor Gerald Butts tweeted "Amazing work #TeamTrudeau. Breathtaking really."
The Conservatives weren't the only party that appeared headed for a crushing defeat. The third place left-leaning New Democratic Party's fall was highlighted in Quebec, where it had the majority of its seats.
Radio Canada projected it would end up with just seven seats, down from 54 in the last Parliament.
The Liberals' win marks a swing toward a more multilateral approach in global politics by the Canadian government, which has distanced itself from the United Nations in recent years.
The former teacher took charge of the party just two years ago and guided it out of the political wilderness with a pledge of economic stimulus and stirring appeals for a return to social liberalism.
Trudeaumania again?
Born to a sitting prime minister who came to power in 1968 on a wave of popular support dubbed "Trudeaumania," Trudeau will become the second-youngest prime minister in Canadian history and brings an appeal more common in movie stars than statesmen.
Pierre once jumped from a trampoline into the crowd. With boyish good looks, Justin thrusts himself into throngs and puts his hand to his heart when listening to someone.
Selfie requests are so common he happily takes the camera and snaps the photo himself, often cheek to cheek. He is the married father of three young children.
Criticized for being more style than substance, Trudeau has used attacks on his good looks and privileged upbringing to win over voters, who recalled his father's rock-star presence and an era when Canada had some sizzle on the world stage.
Pierre Trudeau, who died in 2000, was in power for 15 years - with a brief interruption - and remains one of the few Canadian leaders to be known abroad.
Single when he took power, the elder Trudeau dated movie stars and models before marrying. He had three boys while prime minister, the eldest of whom now succeeds him in the nation's top office.
Financial market players had praised the Conservative government for its steady hand in economic management, which had spared Canada the worst of the global financial malaise. Trudeau has also promised to raise taxes on high-income Canadians and reduce them for the middle class.
Political pundits have already began to speculate on the makeup of a Trudeau government while pondering what caused the downfall of Harper, 56, who has been criticized for his aloof personality but won credit for economic management in a decade of global fiscal uncertainty.
(Writing by Andrea Hopkins; Editing by Amran Abocar and Alan Crosby)
