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.
Cowell got no shortage of attention. ABCNews.com rounded up nine of his most memorable moments in nine seasons of "American Idol." Remember these?
Cowell Calls Seacrest Sweetheart
Cowell and host Ryan Seacrest have long exchanged banter, often about each other's sexuality. In season 6, they stooped to new lows.
After contestant Chris Sligh's performance, Cowell asked fellow judge Randy Jackson a question, which Seacrest answered.
"You do the links, sweetheart. I'll do the judging," Simon said.
"Don't call me sweetheart," Seacrest shot back. "We don't have that kind of relationship. I don't want that kind of relationship..."
More words were exchanged with Cowell looking exasperated. "You've made this very uncomfortable Ryan, now," he said.
Cowell was always the epitome of cool but he broke a sweat the night Season 2 contestant Marine Josh Gracin challenged him to a push-up contest -- and he lost.
On stage, Cowell did his push-ups on his knuckles. It was an impressive show, but Gracin still beat him. More surprising, though, were the high-waisted slacks he sported that night, which Seacrest, of course, took a swipe at.
Cowell and Paula Abdul Fight, Make up
Cowell famously bickered with and ridiculed fellow judge Paula Abdul. But like a pair of siblings, they clearly cared for one another.
It wasn't so clear, though, during one heated debated during Hollywood Week on Season 2. When the two fought over which contestants to cut, Abdul called over producer Nigel Lythgoe. When Abdul tried to explain the problem, Cowell interrupted: "Don't talk about me, talk to my face."
"Simon, I don't have a problem saying anything in front of your face," Abdul said, turning to face Cowell.
"He can't stand someone," she told Lythgoe. Then, looking at Cowell, "There, I said it in front of your face."
Ultimately, Abdul couldn't save her pick, but the judges hugged and made up to the applause of the other contestants in the audience.
Cowell and Abdul's Affair
After two seasons of on-air bickering, Cowell and Abdul exchanged a wet kiss during the 2003 "Idol" season finale. Of course, it was just a skit.
In the taped sketch, the judges played footsie, drank champagne and shared strawberries and whipped cream during a romantic dinner. Cowell realized it was all a dream when he woke up -- with Jackson beside him.
Cowell's Eye Roll Causes Controversy
With one look, Cowell could make clear exactly how he felt. Take for instance, his famous eye rolls. While Season 6's Chris Richardson defended his "nasally" singing, Cowell rolled his eyes.
Unfortunately, this was just as Richardson, a Virginia native, was giving a shout-out to the students of Virginia Tech after the mass shooting that occurred there. A badly timed camera switch caught the eye roll and Cowell had some 'splaining to do.
Cowell Says Sorry to Katharine McPhee
Even Cowell knew when he had taken one of his critiques too far.
When the judge came under fire for eviscerating Season 5 runner-up Katharine McPhee's performance of "I Have Nothing," Cowell issued an unprecedented apology to McPhee.
"I want to apologize because I don't think this person got the right appraisal," Cowell said. "I watched it back and I thought we were unfair. It was a good performance."
It wouldn't be Cowell's last apology.
Cowell Apologizes to David Cook
In 2008, he apologized to Season 7's David Cook before Seacrest delivered the results of the final.
"I went back home to watch it. It wasn't quite so clear cut as we called it. And in fact, David, I will take this opportunity to apologize, because I think I was verging on disrespectful with you, and I don't think you deserved that," Cowell told a stunned Cook.
Good thing, too, since Cook was declared the winner after that.
'Bigger Stage' for Cowell
Some of Cowell's most scathing remarks came during the audition rounds. One of the the most memorable was during the casting of Season 5.
After giving plus-size contestant Mandisa a golden ticket to Hollywood, Cowell couldn't resist a quip after she left the room. "Do we have a bigger stage this year?" he said.
"She's like Frenchie," Abdul added, referring to another former plus-size contender.
"Forget Frenchie, she's like France," Simon said.
Later, Mandisa, who dropped 75 pounds after "Idol," admitted seeing Cowell's comments was her "worst fear come true."
Cowell Hugs a Contestant
Cowell wasn't always so mean. In recent years, Mr. Mean has even gotten a little soft. Blogger Santilli chalks some of that up to boredom. "He's going through the motions," she said. "I'm going to miss Simon at the top game."
But even a softer Cowell is memorable to watch. Last season, when Renaldo LaPuz, from Reno, Nevada, auditioned for the judges, he wore a white suit, silver cape and white feather that that bore Cowell's name.
As expected he sounded awful, but his original song, "We're Brothers Forever," touched something in Cowell, who embraced him before letting him down easy.
"You are very entertaining. ... I actually like you," he told LaPuz, reducing the 44-year-old to tears.