He is the son of an Italian immigrant to Argentina who trained as a chemist, but has now made history by becoming the first Pope from outside Europe in more than a millennium.
Jorge Mario Bergoglio also becomes the first Jesuit leader of the Catholic Church and the first from the Americas.
He is regarded as a humble man who has spent almost his entire career at home in Argentina, and he is said to have denied himself the luxuries previous cardinals in the Argentine capital Buenos Aires enjoyed.
Bergoglio, 76, often took the bus to work, cooked his own meals and regularly visited the slums ringing Argentina's capital. He was said to have received the second most votes after Joseph Ratzinger in the 2005 papal election, and has long specialised in the kind of pastoral work some say is an essential skill for the next Pope.
It is thought Bergoglio will encourage the church's 400,000 priests to take to the streets to capture more souls, said official biographer Sergio Rubin.
He is most comfortable taking a low profile, and his personal style is said to be the antithesis of Vatican splendour. "It's a very curious thing: When bishops meet, he always wants to sit in the back rows. This sense of humility is very well seen in Rome," Mr Rubin said.
Born in Buenos Aires, one of five children of an Italian railway worker and his wife, he trained as a chemist but went on to teach literature, psychology, philosophy and theology before taking over as Buenos Aires's archbishop in 1998.
He became a cardinal in 2001, when the Argentinian economy was collapsing, and won respect for blaming rampant capitalism for impoverishing Argentines.
His legacy as cardinal includes efforts to repair the reputation of a church that lost many followers by failing to openly challenge Argentina's dictatorship from 1976 to 1983. During that period victims and their relatives often told priests in the Jesuit Order he led about torture, killings and kidnappings by the state.
Under Bergoglio's leadership, Argentina's bishops issued a collective apology in October 2012 for the church's failure to protect its flock. But the statement blamed the era's violence in roughly equal measure on both the junta and its leftist enemies.
The apology came far too late for some activists, who accused Bergoglio of being more concerned about the church's image than aiding the many human rights investigations of the Kirchners' era.
Bergoglio twice invoked his right under Argentine law to refuse to appear in open court, and when he eventually did testify in 2010, his answers were evasive, human rights attorney Myriam Bregman said.
At least two cases directly involved Bergoglio. One examined the torture of two of his Jesuit priests – Orlando Yorio and Francisco Jalics – who were kidnapped in 1976 from the slums where they advocated liberation theology.
Yorio accused Bergoglio of effectively handing them over to the death squads by declining to tell the regime that he endorsed their work. Jalics refused to discuss it after moving into seclusion in a German monastery.
Both men were freed after Bergoglio took extraordinary, behind-the-scenes action to save them – including persuading dictator Jorge Videla's family priest to call in sick so that he could say Mass in the junta leader's home, where he privately appealed for mercy.
His intervention may have saved their lives, but Bergoglio never shared the details until Mr Rubin interviewed him for the 2010 biography.
But Ms Bregman said Bergoglio's own statements proved church officials knew early on the junta was torturing and killing its citizens, and yet publicly endorsed the dictators.
"The dictatorship could not have operated this way without this key support," she said.
Later, there was little love lost between Bergoglio and president Fernandez: the president stopped attending his annual "Te Deum" address, when church leaders traditionally tell political leaders what is wrong with society.
The cardinal could not stop Argentina becoming the first Latin American country to legalise gay marriage, or stop President Fernandez promoting free contraception and artificial insemination.
His church had no say when the Argentine Supreme Court expanded access to legal abortions in rape cases, and when Bergoglio argued gay adoptions discriminate against children, Fernandez compared it to "medieval times and the Inquisition."
Like other Jesuit intellectuals, Bergoglio, who had a lung removed due to infection as a teenager, has focused on social outreach.
Catholics are still said to be buzzing over his speech last year accusing fellow church officials of hypocrisy for forgetting that Jesus Christ bathed lepers and ate with prostitutes.
"In our ecclesiastical region there are priests who don't baptise the children of single mothers because they weren't conceived in the sanctity of marriage," Bergoglio told his priests. "These are today's hypocrites. Those who clericalise the Church, those who separate the people of God from salvation."
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