It took months for Fallon to settle down and get his bearings, but it started to become apparent he was bottling beaucoup magic on Late Night.
Fallon was regularly getting his celebrity guests to compete in insane rec-room events, playing beer pong with Helen Mirren, taking on Ryan Reynolds in a water fight, challenging Tom Cruise to a wreath toss, go kart racing with Chris Hemsworth, facing off with David Beckham in egg Russian roulette (the yolk was on the soccer star).
There were cunning spoofs of cultural touchstones like Breaking Bad, Game of Thrones, and Downton Abbey.
Fallon rolled out a deep and celebrated roster of musical impressions, eerily echoing icons like Bruce Springsteen, Neil Young, Eddie Vedder, and Bob Dylan.
He created some inspired song-and-dance numbers with his brother from another mother, favored collaborator Justin Timberlake, instant classics like "The History of Rap" and "The Evolution of End Zone Dancing."
Jimmy was throwing an awful lot of stuff at the wall. And almost all of it was sticking. Not only were these skits popular on air, but they were enjoying a vital second life as videos.
"Jimmy found the sweet spot between television and the Internet," says Colin McGuire, editor and columnist at PopMatters.com. "Because he's very well-connected and very well-liked, he could bring in people like Springsteen. He has the resources to create these great viral videos. They knew from the beginning they would cater to an online audience as much if not more than the television audience."
Given that track record, the prospect of taking over the more popular Tonight Show while NBC's promotional machinery, already in Olympic overdrive, is pushing him hard? Cake.
"Jimmy never set out to make a show that only worked at 12:30 a.m.," says his executive editor, Josh Lieb. "He always thought he was making The Tonight Show. 11:30 p.m. is a more elevated time slot. We'll get more people watching, more prestigious guests."
Sounds as if we're in for the same show at a different hour.
"Perhaps with a little more gloss," Lieb says. "We'll be on right after the news, so we may have to address the news a little more. But it won't be a shock to the system."
It's hard to imagine applying a coat of varnish to Fallon. A big part of his appeal is the manic, almost adolescent energy he radiates. His puppy-dog enthusiasm is kind of irresistible.
"He approaches his desk with a wide-eyed, fan-boy thing," says Steve Morrison, cohost of the morning team Preston & Steve on WMMR-FM (93.3). "He's got this infectious zeal. Given time, I think he'll win over the audience that embraced Leno."
Speaking of Jay Leno, many industry analysts are still puzzled by NBC's hurry to rush him off the stage.
"It's risky anytime you remove the top-rated host from a program," says Brian Stelter, moderator of CNN's Reliable Sources. "You look at this and scratch your head why NBC would do it.
"In television, when you make a change, you take a hit. Some viewers never come back," continues Stelter. "As much as I'm eager and excited [to see Fallon's show], I do wonder if once again it means [ Tonight] won't be as big as it used to be."
Certainly the passing of the baton from Leno to Fallon has not engendered the type of national preoccupation that attended Johnny Carson's retirement in 1992.
"Back then, when the show went from Carson to Leno, you couldn't escape the sense that this was a sea change," says Mark Lashley, a communications professor at La Salle University. "When Leno left, there was some sadness among his fans, but these days we just don't see The Tonight Show as the cultural institution it was back in the network era."
Certainly at 39, Fallon is a host for a new generation, with a fresh and forward vision for the show, a much more contemporary index of pop references, and a true appreciation for social media and Internet synchronicity.
But in some respects, he resembles his most famous forebear.
"Fallon and Carson are taking over the show at about the same age," says Ron Simon, curator for New York's Paley Center for Media. "They're both college-educated and studied what has gone on before them.
"Carson wrote a thesis on 'What Makes Comedy Work,' " says Simon. "Both of them know comedy inside out and can take what came before and can add something uniquely modern to it. That's obviously something Leno didn't have."
Maybe a Carson comparison is inevitable for anyone taking over The Tonight Show.
"It's kind of what Miles said about Louis Armstrong: 'Everybody is playing Pops,' " says Kevin Eubanks, longtime Tonight Show bandleader. "If you're in late night, to some extent, you're always doing Johnny."