Robert Prevost’s brother, John, said a neighbor once told him he’d be pope.

Who is Pope Leo XIV?The 69-year-old Chicago native is the first American pope and is seen as a diplomat in the church.

When Robert Prevost was in the first grade, his neighbor told him he would be the first American pope, his brother told ABC News.

On Thursday, that prophecy came true, when Prevost, a 69-year-old cardinal, was elected to be the 267th pontiff — and the first from the United States.

Before he was Pope Leo XIV, Prevost grew up the youngest of three brothers in the South Chicago suburb of Dolton.

He always wanted to be a priest, his older brother, John Prevost, told ABC News outside his home in Illinois on Thursday.

“He knew right away.

I don’t think he’s ever questioned it. I don’t think he’s ever thought of anything else,” John Prevost said.

As a child, Pope Leo XIV “played priest,” John Prevost said. “The ironing board was the altar.”

The pope is a White Sox fan, his brother confirmed. “He’s a regular, run-of-the-mill person,” he said.

Leo started to emerge as a frontrunner for the papacy in the days before the conclave began, according to the Rev. James Martin, a papal contributor to ABC News.

John Prevost said he spoke to his brother on Tuesday, before the cardinals went into the secretive conclave, and told his younger brother that he also believed he could be the first American pope. At the time, his younger brother called it “nonsense” and “just talk,” saying, “‘They’re not going to pick an American pope,” John Prevost said.

“He just didn’t believe it, or didn’t want to believe it,” John Prevost said.

John Prevost said he expects his brother will follow in the late Pope Francis’ footsteps as a voice for the disenfranchised and poor.

“I think they were two of a kind,” John Prevost said. “I think because they both were in South America at the same time — in Peru and in Argentina — they had the same experiences in working with missions and working with the downtrodden. So I think that’s the experience that they’re both coming from.”

Louis Prevost, the eldest of the three Prevost brothers, was feeling under the weather and lying in bed at his home in Florida when the big moment came.

“My wife called to tell me there’s white smoke from the chapel,” he said.

Louis Prevost said he tuned in to the live broadcast of the Vatican announcement.

“They started reading his name, and when he went, ‘blah, blah, blah, Roberto,’ immediately I knew — that’s Rob,” he said. “I was just thankful I was still in bed lying down, because I might have fallen down.”

Louis Prevost said he got out of bed and started “dancing around like an idiot.”

“It’s just incredible,” he said. “I’m suddenly wide awake and feeling wonderful.”

He described his brother as “down to earth,” someone who has a good sense of humor and is “smart as a whip.” He loved his work as a missionary in Peru and being with the people, and through his work with the Vatican has traveled the world, Louis Prevost said.

“I thought I had done traveling in the Navy, but, my God, he blew me away,” he said.

His brother surmised that global experience may have stood out to the other cardinals in electing him pope.

Louis Prevost said his brother seemed to always know his calling, and that as young as 4 or 5, the family knew he was destined for great things in the Catholic Church. When his brothers were playing cops and robbers, Leo would “play priest” and distribute Holy Communion with Necco wafers, Louis Prevost said.

“We used to tease him all the time — you’re going to be the pope one day,” he said. “Neighbors said the same thing. Sixty-some years later, here we are.”

Leo started to emerge as a frontrunner for the papacy in the days before the conclave began, according to the Rev. James Martin, a papal contributor to ABC News.

John Prevost said he spoke to his brother on Tuesday, before the cardinals went into the secretive conclave, and told his younger brother that he also believed he could be the first American pope. At the time, his younger brother called it “nonsense” and “just talk,” saying, “‘They’re not going to pick an American pope,” John Prevost said.

“He just didn’t believe it, or didn’t want to believe it,” John Prevost said.

John Prevost said he expects his brother will follow in the late Pope Francis’ footsteps as a voice for the disenfranchised and poor.

“I think they were two of a kind,” John Prevost said. “I think because they both were in South America at the same time — in Peru and in Argentina — they had the same experiences in working with missions and working with the downtrodden. So I think that’s the experience that they’re both coming from.”

Louis Prevost, the eldest of the three Prevost brothers, was feeling under the weather and lying in bed at his home in Florida when the big moment came.

“My wife called to tell me there’s white smoke from the chapel,” he said.

Louis Prevost said he tuned in to the live broadcast of the Vatican announcement.

“They started reading his name, and when he went, ‘blah, blah, blah, Roberto,’ immediately I knew — “that’s Rob,” he said. “I was just thankful I was still in bed lying down, because I might have fallen down.”

Louis Prevost said he got out of bed and started “dancing around like an idiot.”

“It’s just incredible,” he said. “I’m suddenly wide awake and feeling wonderful.”

He described his brother as “down to earth,” someone who has a good sense of humor and is “smart as a whip.” He loved his work as a missionary in Peru and being with the people, and through his work with the Vatican has traveled the world, Louis Prevost said.

“I thought I had done traveling in the Navy, but, my God, he blew me away,” he said.

By Alex Perez and Meredith Deliso