Muldoon was born in San Pedro, California, the son of Deanna, a homemaker, and William Patrick Muldoon II, a personal injury lawyer. He is of Irish descent on his father’s side and of Croatian descent on his mother’s side. He attended Loyola High School, a Jesuit school. Muldoon graduated in 1991 from the University of Southern California, where he was a member of the Sigma Chi fraternity and played tight end on the USC Trojans football team.

Muldoon was the first actor to play the role of Austin Reed on Days of Our Lives, a role which he played from 1992 to 1995 and September 2011 to July 2012. He had a role in the popular teen television series Saved by the Bell in 1991. Next he played the popular villain Richard Hart, on the prime time soap opera Melrose Place, from 1995 to 1996. He was the only actor ever to have an exclusive development deal with Spelling Entertainment. He starred in Black Cat Run which also marked the debut of director DJ Caruso (Disturbia, I am Number Four) with Frank Darabont. He also played Edmund in the Patsy Rodenburg production of King Lear. He played Zander Barkalow in the Paul Verhoeven-directed Starship Troopers. He has starred in many popular television movies and has been a regular in many films for the Lifetime & Hallmark channels. In the past decade he has also become a prolific film producer.

In 2015, he produced and starred in Badge of Honor starring Martin Sheen and Mena Suvari. In 2019, he produced and starred in Arkansas (directed by Clark Duke), which also starred Vince Vaughn, John Malkovich, and Liam Hemsworth. In 2019, he produced and starred in The Comeback Trail, starring Robert De Niro, Tommy Lee Jones, and Morgan Freeman. Directed by George Gallo.