Lee DeWyze claimed the title as the ninth "Idol" over runner-up Crystal Bowersox, but it was Cowell who claimed the night as his own.
The two-hour show included several montages of Cowell's more memorable tongue-lashings, a roasting by comedian Dane Cook and a familiarly untethered speech by former judge Paula Abdul.
"I've loved all the fun we've had together," Abdul told Cowell. "'American Idol's' not gonna be the same without you. But as only I can tell you, it will go on."
Seven of the eight previous Idol winners appeared to serenade the outgoing capo.
"I didn't think I was going to be this emotional and I genuinely am," he told the crowded room. "Everybody asks who's going to replace me, who going to be the next judge. The truth is ... you guys are the judge of this show and you've done an incredible job over the years."
We'll miss Cowell's flat-top and black and white fitted tees, the eye rolling and quick quips. Mostly, we'll miss his critiques.
Mr. Mean or Mr. Nasty, as he's been called, transformed reality television with his stinging criticism, making it mandatory to have at least one "mean" judge on a panel. There'd be no Piers Morgan ("America's Got Talent") or Bruno Tonioli ("Dancing With the Stars") without him.
"He created that kind of a judge," Us Weekly senior editor Ian Drew told ABC News. "And every reality show that's come after, there's always been a Simon in the mix. There's always been the one who is going to tell it like it is, who is going to do it with biting wit and humor, and to play a bit of a villain."
Cowell leaves behind a repertoire of riffs, which he usually prefaced with "I'm sorry, but..." or "I don't mean to be rude, but..." He turned the words "karaoke," "cabaret" and "lounge singer" into insults. "That was terrible" and "ghastly" were among his favorite quips.
Some of his best zingers came from the auditions, where he told one contestant: "If you had lived 2,000 years ago and sung like that, I think they would have stoned you."
Another, in season one, set the tone for what would follow over the next eight seasons: "Are you taking singing lessons? Who's your teacher? Do You have a lawyer? Get a lawyer and sue her."
Cowell's unsparing honesty, often spot-on, made him beloved to "Idol" audiences. Many say the show will not survive without him.
"Basically, it really means the end of the show," Drew said. "I mean it really was pretty bad this season, and it can't get any worse. Basically, I mean it can only go down from here. Basically, Simon's gone, the flavor of the show is gone, it's all going to get transferred to his new show, "X Factor."
Though Howard Stern and even Madonna have been touted as possible replacements, Cowell's shoes will be hard to fill.
"They're going to have to get somebody like Diddy or Jay Z with marquis value," said MJ Santilli, who writes about "Idol" on MJ's Big Blog. "People know who they are and they have industry experience."
New judge Ellen DeGeneres has the name recognition, Santilli said, but no musical credibility. "She doesn't have a lot to say," she said.
To survive, the show will need to find someone who carries as much weight as Cowell did.
"They're going to need somebody who's tough, maybe not mean but tough -- not necessarily nasty, but commands respect and attention," Santilli said.