That cheeky move led to a seven-year tour as Obama's lead speechwriter, an assignment that ends March 1 as Favreau considers trying his hand at another form of drama — as a screenwriter, perhaps in Los Angeles.
The departure subtracts a vivid personality from the president's operation, defined since the beginning by Obama's spoken words and the team that wrote them.
After Favreau landed in the White House four years ago, he became the most recognizable in a coterie of young staffers. Sporting aviator sunglasses and a buzz cut, he occasionally lit up social media with his antics.
People magazine named him one of the world's most beautiful people. He went out with actress Rashida Jones, best known for her role in "The Office." One night, as he and some friends played a shirtless game of beer pong in Georgetown, someone snapped a photo that ended up on the blog FamousDC, with the headline: "White House Gone Wild."
But about the writing, Favreau was always serious, telling peers it was a solemn responsibility to remain in sync with the president's thinking.
"When they're working together, it's like watching two musicians riff," said David Axelrod, Obama's longtime advisor. "Jon's stamp is on all of the great speeches, from 2005 until now."
Favreau will turn over his seat to Cody Keenan, a Chicago native who is taking the lead on writing the State of the Union address. Keenan is an original member of the team of twentysomethings that Favreau assembled for a tough assignment: writing for a writer with exacting standards.
Favreau declined Monday to discuss his departure.
In a statement, Obama said, "He has become a friend and a collaborator on virtually every major speech I've given in the Senate, on the campaign trail and in the White House."
They didn't start off as collaborators. Obama was an Illinois state senator running for the U.S. Senate when they met in 2004. He was preparing to deliver the Democratic National Convention speech that would launch his national career. Favreau was working as a junior speechwriter for the party's presidential nominee, Sen. John F. Kerry (D-Mass.), who is from Favreau's home state.
Kerry's staff had spotted an overlap between Obama's speech and the one their boss planned to deliver, and they sent Favreau to tell Obama to trim his text.
"It was an unbelievably cruel thing to do, to send the 23-year-old in to do that job," Axelrod joked.
After Obama was elected to the U.S. Senate, he hired Favreau. Favreau then moved to Obama's 2008 campaign and into the White House, where he earned a reputation as someone who could write speeches and parry with senior officials and Cabinet secretaries who wanted to put their fingerprints on the work.
If there were any doubts about him, Favreau quickly dispelled them when he wrote the first inaugural address and the president's healthcare speech to Congress, said David Plouffe, a longtime Obama advisor.
"Jon wasn't going to come in with a draft that was not Barack Obama-like," Plouffe said. "The president never has to worry that he's going to get something and have to say, 'This isn't my voice.'"
Keenan is known for his handling of heartbreak and sadness. He was the lead writer on Obama's speech at the Tucson memorial after the shooting of then-Rep. Gabrielle Giffords (D-Ariz.).
Favreau plans to stay in Washington for a while, but he has often told friends that he wants to pursue screenwriting, as did former Obama speechwriter Jon Lovett, the co-creator of the new comedy "1600 Penn."
His time in the White House should serve Favreau well, Plouffe said.
"He can write comedy, history, drama, suspense," he said. "He's got the whole range."