Born in Louisiana, Harmon attended Rockwall High School in Rockwall, Texas, and graduated in 1982. He went on to study at Texas A&M University on a football scholarship. Harmon was one of the first to know that Mark Consuelos and co-star Kelly Ripa were dating and had secretly gotten married in Las Vegas.

Harmon portrayed Del Henry on the program All My Children from 1994 to 1995. He has made appearances on Baywatch Nights and Acapulco H.E.A.T.. Harmon told Soap Opera Digest about his guest stint on H.E.A.T.: <.. I hung out with John McCook the whole time and I kept watching all these people from other countries asking for his autograph. I'm thinking to myself, 'Who the hell is this guy?' So one night, we were drinking Coronas and I asked him why he had so many fans and he explained how B&B was so popular around the world. I had no idea. The more he talked about it, the more I was thinking, 'I want to be on that show.'... The first time I drove into the parking lot, there was John, just pointing at me and laughing. He asked what I was doing here and I told him that I was Thorne. That's when he told me that he was going to be playing my dad. That was just too wild."