"Issues aside, 20 years at #1 is a remarkable achievement. Congratulations and best wishes to @jayleno on an incredible run," Kimmel, 46, tweeted on Thursday, just a few hours before Leno tapes his final episode of his NBC late-night talk show with guests Billy Crystal and Garth Brooks.
It's a nice sentiment, given the nasty things Kimmel has said about Leno in the past.
During the great late-night shakeup of 2010, Kimmel devoted an entire episode to impersonating Leno by donning a floppy gray wig and wearing an oversized prosthetic chin. "Hello! My name is Jay Leno. You might have known, I'm taking over all of the shows in late night. Even this one," Kimmel said in his monologue. "Great to be here on ABC. You know what ABC stands for? Always Bump Conan. That's right. Anyway, Conan O'Brien today announced he's leaving NBC. He released a statement that said, 'I won't participate in the destruction of the Tonight Show.' Fortunately, though, I will."
Leno, 63, responded to Kimmel's attention ploy by inviting him to do a satellite interview. Two days later, Kimmel appeared on The Tonight Show. "What's the best prank you ever pulled?" Leno asked.
Kimmel jokingly referred to O'Brien and said, "I think the best prank I ever pulled was, I told a guy that five years from now I'm gonna give you my show and then when the five years came, I gave it to him and then I took it back almost instantly. It was hilarious...I think he works at Fox or something now."
"Ever ordered anything off the TV?" Leno asked. Kimmel fired back, "Like NBC ordered your show off the TV?" Leno then inquired, "Is there anything that you haven't hosted that you want to host?" After a brief pause, his younger competitor replied, "Oh, this is a trick, right? Where you get me to host The Tonight Show and then take it back from me?"
Kimmel later called the back-and-forth his "proudest moment"; Leno later described it as a "sucker punch."
"If I made any mistake, the mistake I made was thinking Jay would have anything other to say than what was written on the cue cards," Kimmel said of the segment. "I figured he'd get into it and mix it up. You know, at one time he was a comedian. But he just stood there, kind of reading through the questions like a robot."
During an August 2012 appearance at the 92nd Street Y, a local community center on New York City's Upper East Side, Kimmel was asked how he felt about Leno. "F--k him," the Brooklynite replied.
"People, it turns out, really don't like Jay Leno," Kimmel added. "We have not spoken since that time."
Two months later, Kimmel dissed the veteran in a New York article: "Everyone is better than Leno." He went a step further in January 13 when he was profiled in a Rolling Stone cover story.
He admitted to being bitter that Leno outmaneuvered rival David Letterman, who was seen as the heir apparent to Johnny Carson in 1992. "Remember, he stole The Tonight Show from Dave," Kimmel said. The twice-married comic also sniped, "Leno hasn't been a good stand-up in 20 years."
Kimmel had been angry with Leno long before the late-night shakeup. "As a comedian, you can't not have disdain for what he's done: He totally sold out. He was a master chef who opened a Burger King."
Leno, for his part, kept quiet about Kimmel's mean-spirited comments. Speaking to The Hollywood Reporter in February 2014, he said, "I don't get into public feuds with other comics."
He added that there's nothing to gain from responding to Kimmel. "Rich people whining and complaining? Shut up. You make more money than 99 percent of the population and you're complaining and whining. My job is to go out there and be a comedian." Leno noted that there's a difference between his so-called feud with Kimmel and his "interesting relationship" with Letterman.
"Is it a joke? Is it funny? You know, Letterman and I have had a fun relationship because when Letterman says something, it's funny," Leno explained. "I learned from Dave the subtleties of doing a joke, and I think he learned from me how to really sell a joke. So there was always a mutual admiration, and we always made each other laugh."
Leno's final Tonight Show airs Thursday, Feb. 6, at 11:35 p.m. EST on NBC.
(E! and The Tonight Show are both part of the NBCUniversal family.)