Zardari has spent a total of 11 years in jail on charges ranging from corruption to murder, though he was never convicted of the crimes and has maintained his innocence. President Pervez Musharraf cleared him of all corruption charges last year and allowed him and Bhutto to return to Pakistan, ending years of exile.
Benazir Bhutto was campaigning for upcoming general elections in December 2007 when she was assassinated in a suicide attack following a public speech.
In the wake of her death, Zardari took over the leadership of her Pakistan People's Party, and constructed a network of political allies who threatened Musharraf with impeachment -- pressure that eventually led him to resign.
Zardari has promised to be tough on militants and help unify and stabilize the country.
"I will work to defeat the domestic Taliban insurgency and to ensure that Pakistani territory is not used to launch terrorist attacks on our neighbors or on NATO forces in Afghanistan," Zardari said in an editorial before his win.
Like his wife, Zardari is generally considered to favor Western powers and support the U.S.-led war on terror in the region, according to media reports. In February, before parliamentary elections, he said the PPP seeks to make Pakistan part of the solution for the world, not the problem.
But doubts continue about Zardari's character. The country's leading newspapers on Sunday called for Zardari to prove he is trustworthy after he easily won a vote Saturday in the country's parliament and provincial assemblies to clinch the presidency.
"There have been more controversial presidents in the past," Dawn, an English-language daily, said in an editorial, reported the Agence France-Presse. "But none has been as controversial as Mr. Zardari at the time of assuming office."
Zardari shot back at critics after his sweeping victory.
"To those who would say that the People's Party or the presidency would be controversial under our guardianship, under our stewardship, I would say listen to democracy," Zardari said, reported National Public Radio.
Zardari, who was born and raised in Karachi, married Bhutto in 1987 through an arranged marriage. He was the son of a cinema owner, and was thought to be marrying up to the Oxford-educated, politically connected Bhutto.
In 1988, she became Pakistan's prime minister, but after two years in office, she was dismissed on charges of incompetence and corruption. Most of the charges of corruption were directed against Zardari, but Bhutto's handwriting on notes attached to suspicious contracts implicated her as well.
In 1990, Zardari was also accused of strapping a remote-controlled bomb to the leg of a businessman to force him to go into a bank to make a withdrawal for a pay-off.
Zardari was jailed for the charges, but was then released and made minister for investment when Bhutto was re-elected in 1993. In 1996, she was ousted again and he was arrested again, accused of murdering Bhutto's brother, Murtaza, though the case was never proven and he was not convicted.
He and Bhutto were both charged and convicted in a money-laundering scheme involving a Swiss company, which kept him imprisoned until 2004 while she lived in self-imposed exile outside the country. The conviction was later declared a mistrial by Pakistan's Supreme Court.
Despite the fact that he was cleared of corruption charges by Musharraf, Zardari faces some tough critics who are still not convinced he is presidential material. As president, he will also have to deal with a dire economic situation and militant violence in the country's restive tribal belt.
Ahsan Iqbal, a former minister and senior figure former premier Nawaz Sharif's party, told the AFP his first move should be to show he will not continue Musharraf's stranglehold on power.
"Zardari's first test is that as president he facilitates the transfer of Musharraf's powers to parliament," Iqbal said.
And while the people of Pakistan may not be convinced of Zardari's character yet, some seem willing to give him the benefit of the doubt as a last allegiance to their slain hero Bhutto.
"Even his worst enemies couldn't prove anything against him," Javaidur Rehman, a businessman in Multan, told the Associated Press. "Everyone is full of mistakes, but we should not doubt his sincerity. His family has made too many sacrifices."