It was rumoured that Simon could be making an appearance for the finale, but he said last week that a clash of dates meant he may not be able to attend.
But, the X Factor boss turned up to the star-studded farewell show where he joined Randy Jackson and Paula Abdul on stage.
The trio paid tribute to host Ryan Seacrest, and Simon admitted that he was "feeling quite emotional" about the end of the show.
Randy tweeted a picture of the trio too, simply saying: "The originals."
Simon's former ex, and American Idol correspondent Terri Seymour posted a backstage photo with Simon and his girlfriend Lauren Silverman with the caption: "As if he'd miss the last show!"
Music mogul Simon was an original judge on the US show from 2002 alongside Paula and Randy before leaving in 2010.
American Idol, created by Simon Fuller, was based on the British series of Pop Idol.
The show founded many of today's mainstream breakout stars, and at its height, it drew in an audience of around 36 million viewers per episode. The last season averaged around 11 million viewers.
When the end of Idol was announced last May, Simon said: "We had a blast! Me, Paula, Randy, Ryan - and not forgetting Brian. Thanks to the fans we were able to make some great shows and found some amazing artists.
"Fox took a risk on a brand new show and I want to thank them for giving us the opportunity. They really were great times."
Jennifer Lopez, Keith Urban and Harry Connick Jr., were the final three judges on the show, They watched Trent Harmon be crowned the 15th and final winner of American Idol, leaving La'Porsha Renae as runner-up.
US President Barack Obama kicked off the last ever show with a pre-recorded message congratulating everyone on its 15-year stint on TV.
At the very end of the show, Ryan said: "Good night, America" as the screen faded to black, adding, "for now".
UK fans can catch the final episode of American Idol in full on 4Music, next Monday (11th April) from 8pm.
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