The ceremony occurred about an hour after Jean Chretien, his predecessor and longtime rival, handed in his resignation on Friday.
Martin, 65, immediately named Anne McLellan as his deputy prime minister.
A key loyalist, McLellan will also be responsible for public safety and emergency preparedness, as well as coordinating public security with the United States.
The new PM is a multimillionaire businessman best known for helping eradicate the country's enormous budget deficit in the mid-1990s when he was finance minister.
A decade of rivalry between Martin and Chretien boiled over in June 2002 when Chretien sensationally sacked Martin for plotting against him.
The Liberal leader is on the fiscally conservative side of his party, but how he will govern remains something of a mystery.
Chretien dominated Canadian
politics for more than a decade
The most important influence in his life was his father, Liberal cabinet minister Paul Martin Sr, the left-leaning architect of Canada's post-war social policy.
Paul Martin Sr tried and failed three times to become Liberal leader, experiences which put enormous pressure on his son to go one better.
"Paul basically has three sides to him: He's his father's son, he's a successful businessman and he's a government minister. When you speak to him you never know which side you'll be facing," said one close friend.
'His father's son'
"Now I'm not for one second suggesting he's unstable in any way, because that isn't true," he added. "It's just that Paul is a very complex individual."
Martin is known for his short temper and his desire to mull over a question from every possible angle before taking a decision.
He also has an impulsive side and admits to throwing stones at the Soviet embassy in Ottawa when still a teenager.
Public expectations are enormous for Martin, who has promised to give more freedom to legislators, spend more on the decaying armed forces, end internal provincial hostility, and repair relations with Washington.
Martin is also interested in foreign affairs and aiding the Third World. He played a large role in the creation of the G20, a grouping designed to give emerging countries more of a say in international affairs.
Former finance minister
Martin and Chretien were the two main players in the Liberal government which took power in 1993 and inherited a deficit of $32 billion.
The country's plight was so great that the Wall Street Journal said Canada had "become an honorary member of the Third World".
"Paul basically has three sides to him: He's his father's son, he's a successful businessman and he's a government minister. When you speak to him you never know which side you'll be facing"
A friend of Paul Martin
However, Martin slashed public spending in his 1995 budget and restored the country's financial reputation.
The cuts badly hurt the state-funded, universal health-care system, which has been short of cash ever since - but Martin was unapologetic.
"Our credibility in the world was so low that the markets were ready to kill us and drive interest rates through the roof... so we had to do it (cut spending). But I've got to tell you, that was not an easy decision," he told John Gray, author of a recent biography.
The budget reinforced his reputation and he tightened his grip on party organisations across the country while gradually gaining the support of most legislators.
But the growing antagonism with Chretien grew too strong to handle and Martin lost his job in June 2002.
He gained his revenge two months later when irate Liberal legislators forced Chretien to announce he would quit by February 2004.