Nick Cannon will also reprise his role as host of the show's 2017 season.
"It was terrific to see Simon in his judge's chair this past season, bringing his unique expertise and enthusiasm to the show, night after night," NBC reality chief Paul Telegdy said in a statement of the America's Got Talent creator.
"His brilliance has made AGT the industry leader that it is today and his ability to evaluate and spot talent is unmatched, so we couldn't be happier to have him seated at the judge's table for the next three years."
Cowell will be returning next year for his second straight season on America's Got Talent. Klum and Brown will be back for their fifth edition, while Mandel will be back for his eighth. Cannon has been hosting the show for nine seasons.
"America's Got Talent is a fantastic and -- most importantly -- fun show both to work on and to watch," Cowell said in a statement.
"NBC have been brilliant partners and the entire team who make this show are one of the best in the world. I've loved this past season and look forward to discovering more great talent together in the years to come."