Cowell, 56, may have a successful record label and television production company now, but he said he had "miserable" months trying to sell houses in London, reported Female First.
"After I left the music industry I was an estate agent for eight months. And I can honestly say it was the most miserable eight months of my life.
"I worked for this really snotty company in Mayfair who hated me and I hated them, and I was so upset that my mum actually said to me 'I've never seen you so depressed.' And luckily I was offered a job back at the music publishing company. I'm glad I tried it because it was horrible," he said.