Pope Leo XIV used to "play priest" when he was a child, according to his brother, who said that he could not be prouder of the new pontiff.
Yesterday Chicago-born Robert Francis Prevost was elected by fellow cardinals to lead the world's 1.4 billion Catholics after a secret conclave in the Vatican's Sistine Chapel.
Speaking to journalists in Chicago, John Prevost said at one point when they were children, a neighbour said Robert would be "the first American Pope".
"He took our mom's ironing board and put a tablecloth over it and we had to go to mass," Mr Prevost said.
"He knew everything. He knew his prayers in Latin. He knew his prayers in English and he did that all the time and he took it totally serious.
"It was not a joke. It was not a game. He was dead serious about it."
Mr Prevost said he was talking to his niece yesterday when the white smoke emerged from the Sistine Chapel chimney. He said he could not believe it when his brother emerged from the balcony as the new Pope.
"We heard our name called, and she screamed and I sort of... oh my gosh this is for real," he said.
"We just listened and watched the people in the crowd and listened to his speeches because he didn't say a whole lot in English. He used Spanish and Italian, so we didn't get a whole lot, but there were some subtitles and then my phone off, off went the iPad, everything went off at the same time."
Mr Prevost added that his brother is "young to be a Pope" and he is active, so "he will be here and there and moving around and I think he will also speak his mind.
"It's hard to put into words. I mean, how can one be more proud of someone who is not only the Pope but the first American Pope.
"The first Pope from Chicago and my brother."
