Born in Argentina, Pope Francis is the first Latin American to lead the Roman Catholic Church, as well as the first Jesuit.
"It seems my brother cardinals went almost to the end of the world" to choose a pope, he told the crowd in St Peter's Square in his first address - a joke which belied his image as the cardinal who never smiles.
Up until 13 March, he was Cardinal Jorge Mario Bergoglio of Buenos Aires.
Analysts did not see him as a favourite for the job of succeeding Benedict XVI and his advanced age - at 76, he is just two years younger than Benedict at the time of his election in 2005 - may have surprised those expecting a younger man as the 266th Pope.
However, he appeals to both Church conservatives and reformers, being seen as orthodox on sexual matters, for instance, but liberal on social justice - through far from being a "liberation theologist".
He was born on 17 December 1936 in Buenos Aires, of Italian descent.
According to his official Vatican biography, he was ordained as a Jesuit in 1969 and went on to study in Argentina and Germany.
He became a bishop in 1992 and Archbishop of Buenos Aires in 1998. At the 2005 conclave, he was seen as a contender for the papacy.
His election took many by surprise in his home city, where many had thought his age ruled him out, says the BBC's Marcia Carmo in Buenos Aires.
But any surprise soon gave way to the jubilant blaring of car horns on the streets.
As Cardinal Bergoglio, his sermons always had an impact in Argentina and he often stressed social inclusion, indirectly criticising governments that did not pay attention to those on the margins of society, our correspondent says.
Francesca Ambrogetti, who co-authored a biography of him, told Reuters news agency that part of his public appeal lay in his "sober and austere" humble lifestyle.
"That's the way he lives," she said. "He travels on the underground, the bus, when he goes to Rome he flies economy class."
In Buenos Aires, he lived in a simple flat in the building of the Archdiocese.
When in Rome, BBC Latin America analyst Eric Camara writes, he often preferred to keep his black robe on, instead of the cardinal's red and purple vest he is entitled to wear.
He is also said to have re-used the cardinal's vest used by his predecessor.
According to a profile in the UK's Guardian newspaper, when he was appointed a cardinal in 1998, he urged Argentines not to travel to Rome to celebrate but to give their money to the poor instead.
According to Ms Ambrogetti, he is a moderate in all things.
"He is absolutely capable of undertaking the necessary renovation without any leaps into the unknown," she said.
"He would be a balancing force. He shares the view that the Church should have a missionary role, that gets out to meet people... a church that does not so much regulate the faith as promote and facilitate it."
For the Church establishment, it will be a novelty to have a Jesuit in charge - members are supposed to avoid ecclesiastical honours and serve the Pope himself.