He later told cardinals over dinner: "God forgive you for what you've done!"
In a phone call to an Italian journalist friend that same night he laughed out loud when she asked whether she should start calling him "Holy Father" -- the standard style for popes -- instead of just "Father" as before.
Quipping was definitely not on the cards with Francis's predecessor Benedict XVI, who rarely strayed from prepared texts and was fond of delivering theological treatises in public, although he was said to reveal an ironic wit in private.
Pope Francis "is very relaxed and behaves in public just like he would in private," said Marco Politi, a Vatican expert, who said the new pontiff seemed "very at ease and very natural" and had decided to just "be himself".
Speaking to pilgrims in St Peter's Square on Sunday, Francis cited a book by German cardinal Walter Kasper, then said: "Don't think I'm plugging my cardinals' books, eh? It's not like that!"
He then recounted the story of an elderly Argentine woman who had told him that if God did not forgive sins, the world would not exist.
"I felt like asking her: did you study at the Gregorian?" he joked.
The Pontifical Gregorian University in Rome is an intellectual training ground for Catholic leaders run by the same Jesuit order Francis belongs to.
The tango-loving Argentine pope's humour shone through the previous day too in remarks to the world's press when he broke off from a prepared text to gaze around a Vatican auditorium and said: "You've worked, eh? You've worked!"
Francis also departed from papal tradition by revealing a few details of the supposedly top-secret deliberations among cardinals that led to his election.
He quipped that the point at which he realised he was about to become the leader of the Catholic world was "when things started to get a bit dangerous".
In more of an inside joke, Francis said one prelate asked why he had not chosen the name Clement XV "to avenge the Jesuits". Clement XIV dissolved the Jesuit order in 1773 and it was only restored more than 40 years later.
On another formal occasion, Francis told the world's cardinals that he and they were all "old" men but that this meant they had wisdom -- "good wine that gets better over the years".
Francis is not the first pope with a sense of humour, although perhaps one of the few who has displayed it so publicly in recent times.
Several Renaissance popes employed jesters -- including Alexander VI, whose dissolute reign is an embarrassing memory at the Vatican.
In the 18th century, Benedict XIV was said to be something of a prankster, and in more recent times John Paul I joked with senior prelates.
The tension of John Paul I's election in 1978 saw one nicotine-deprived cardinal -- some stories say it was an American, others say he was Spanish -- ask the new pope if he could have a cigarette.
John Paul I thought long and hard before replying: "Eminence, you may smoke, just as long as the smoke is white" -- a reference to the smoke signal from the Sistine Chapel that shows a new pope has been chosen.
"If someone had told me I would be pope one day, I would have studied harder," he is reported to have said one day in his tragically short reign.
As another pope, Pius IX, lay dying in 1878, the cardinal vicar of Rome ordered round-the-clock bell ringing and prayers in Rome to speed his recovery.
The pope quipped: "Why do you want to stop me from going to heaven?"