The change at "Tonight," the longest-running and most popular late-night talk show, had been widely reported but not confirmed by the network until Wednesday. NBC reportedly just wrapped up negotiations with Fallon on a contract extension.
Steve Burke, chief executive officer of NBC Universal, said the network is purposefully making the move while Leno is still at the top of the ratings, just as when Leno replaced Johnny Carson at "Tonight" in 1992.
"Jimmy Fallon is a unique talent, and this is his time," Burke said.
Leno, in a statement, offered his congratulations to Fallon.
"I hope you're as lucky as me and hold on to the job until you're the old guy," he said. "If you need me, I'll be at the garage."
Fallon said, "I'm really excited to host a show that starts today instead of tomorrow."
Emotions were mixed among people waiting outside Leno's Burbank, Calif., studio to attend the taping of Wednesday's "Tonight" show.
"We love you, Jimmy!" said Natalie Renfro, 45, of Salt Lake City. But she gave a shoutout to Leno, too: "I'll miss that big chin!"
Ryan Kelly, 39, of Los Angeles, said he's a Leno fan but added that the comedian has "had a good run. ... It's probably time for a fresh face. He's done a good job and I'm sure he'll pop up on TV somewhere else."
As for switching to Fallon, "I'll give it a shot," Kelly said.
Leno couldn't resist a jab at NBC in his monologue Wednesday, even as he as he lauded Fallon as "a hell of a guy" who is going to do a "great job."
"I just have one request for Jimmy: We've all fought, kicked and scratched to get this network up to fifth place. Now we have to keep it there. Jimmy, don't let it slip into sixth!" Leno joked, according to an NBC transcript.
Fallon took a puckish approach in his monologue.
"Welcome! This is 'Late Night with Jimmy Fallon' ... for now," he said. "You guys probably heard the news: I'm going to be taking over the 'Tonight' show next February! But don't worry. Until February, our focus is right here on whatever this show is called."
On his "Late Show" Wednesday on CBS, David Letterman feasted on NBC's announcement.
"Jay Leno now is being replaced, and this is the second time this has happened," he said in his monologue. "I mean, it's crazy. He's being replaced by a younger late-night talk show host what could possibly go wrong? Honestly. They had pretty good luck with this in the past."
Later, Letterman offered a backhanded salute to Leno that wished him well with his stand-up appearances.
"But good luck to Jay. I know he'll be out on the road, getting it done and taking care of business. And congratulations on a nice long run there at the 'Tonight' show, if, in fact, you're not coming back," Letterman said, according to a CBS transcript.
He devoted his Top 10 list to "things we'll miss about Jay Leno," including this at No. 4: "Can't remember the name of the bit, but it's the one where Jay is walking."
NBC has been quietly building a new studio for Fallon at its Rockefeller Center headquarters. "Tonight" began in New York in the 1950s, but Carson moved it to California in 1972. Starting next year, Fallon, Letterman, Jon Stewart and Stephen Colbert will tape late-night shows in New York. ABC's Jimmy Kimmel and TBS's Conan O'Brien will be the top California-based shows.
"The 'Tonight' show will bring even more jobs and economic activity to our city, and we couldn't be happier that one of New York's own is bringing the show back to where it started, and where it belongs," said New York Mayor Michael Bloomberg.
New York state recently added a tax credit in its budget that seemed designed specifically to benefit NBC's move east with "Tonight."
While a storied part of television tradition, the network late-night shows find themselves with much more competition now with cable programs like "Adult Swim," smaller talk shows hosted by Chelsea Handler and the Comedy Central duo of Stewart and Colbert, and a device a large number of people take that time to watch programs they had taped earlier on their DVRs.
NBC is worried that Kimmel will establish himself as a go-to late night performer for a younger generation if the network doesn't move swiftly to install Fallon. ABC moved Kimmel's time slot to directly compete with Leno earlier this year.
But the move also has the potential to backfire with Leno's fans, who did not embrace O'Brien when Leno was temporarily moved to prime time a few years ago.
"The guys at NBC are not totally stupid and are not going to shoot themselves in the foot," said Gary Carr, senior vice president and executive director of national broadcast for the ad buying firm TargetCast. "I think it's a good move for them long-term. But it may have short-term ramifications."
NBC has long prided itself on smooth transitions, but that reputation took a hit with the short-lived and ill-fated move of O'Brien to "Tonight" and Leno to prime time. In morning television, the "Today" show has taken a ratings nosedive in large measure because of anger at how Ann Curry was treated when she was ousted last year as Matt Lauer's co-host.
The Leno-Fallon changeover didn't begin smoothly. Leno had been cracking jokes about NBC's prime-time futility, angering NBC entertainment chief Robert Greenblatt, who sent a note to Leno telling him to cool it. That only made Leno go after NBC management much harder.
The first public effort toward making the transition smooth came Monday night, when Leno and Fallon appeared in a comic video making fun of the late-night rumors. It aired in between each man's show.
John Dawson, general manager for five NBC affiliates that have extensive reach throughout Kansas, said it will be difficult to give up a program that wins its time period by 33 percent.
"Jay has always been a great friend to the affiliates," he said. "For that alone it will be hard to give up."
But he said he believes in Fallon and in NBC's corporate owner, Comcast Corp., the nation's largest cable company.
"Comcast certainly knows how to launch entertainment programming," Dawson said.