Muldoon was born in Los Angeles, 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 based on his day-time popularity. He starred in "Black Cat Run" which also marked the debut of Director DJ Caruso ( Disturbia, I am Number Four ) He also played Edmund in the Patsy Rodenburg production of King Lear. He is perhaps most well known for his roles in 1997's Starship Troopers and 2007's Ice Spiders. He has starred and continues to star in many television movies a regular in many films for the Lifetime & Hallmark Channels.
He is the lead singer and the Guitarist for the rock band The Sleeping Masses. In 2010, they released their music video "The Woman Is the Way" with OceanFall Agency and begun working with agent Leon Mitchell in the UK for The Sleeping Masses. The song is the end title song from the movie Powder Blue starring Jessica Biel.
In 2015, he produced and starred in Badge of Honor starring Martin Sheen and Mena Suvari.
