Walsh, 61, has now signed his contract and in an interview on Ireland's 2FM, he said: "I knew I was doing the show but I just signed the deal and I'm very happy with it.
"At my age you have to keep working."
Rumours have abounded that pop star Cheryl Cole, who is returning after a three-year absence, was not happy about working with Walsh.
Judge and producer Cowell previously said: "I'd love Louis to come back and do the show - it wouldn't be the same without him.
"Other people have different views, so we have to agree on it."
Walsh will be taking the third seat on the judging panel, leaving one space open. Spice Girl Mel B, who has previously made guest appearances on the show, is tipped as the frontrunner for the remaining seat.
Talking of the remaining seat, Louis said: "It has to be someone I like. I'd love it to be Sharon Osbourne but I don't think it's going to be her.
"There are all sorts of rumours going around. I think it could be Mel B from the Spice Girls. I love her and she's quite mad.
"She also did both the American and the Australian versions of X Factor."