The future king halted a fight between the 37-year-old actor and another man during an evening out at an exclusive London bar.
Mellor tells Britain's Daily Star newspaper, "I'm not a violent man but I was in a nightclub and I got into a scuffle with another man. The other person got hold of me from behind and said, "He's not worth it, Will." And it was Prince William.
"And then before I knew it there was a secret service (agent) closing in. They thought I was having a scuffle with him, but I wasn't. I got thrown out of the club. On the way out I said, 'Cheers Will'. It still haunts me."