Mulroney co-starred in the 1988 western Young Guns, for which he is possibly best known, in which he played "Dirty Steve" alongside Emilio Estevez. P", the boyfriend of star "Maggie" (played by Bridget Fonda) in Point of No Return.
In 1994 he starred with Mickey Rourke in The Last Outlaw, which has since become a cult classic among western genre fans.
Mulroney played the main antagonist of NBC's Crisis, Francis Gibson. His mother, Ellen, was a homemaker and amateur actress originally from Manchester, Iowa, and his father, Michael Mulroney, was a law professor (and amateur racecar driver) at Villanova University School of Law, originally from Elkader, Iowa.
He played cello in the song "Place Your Hand" from Melissa Etheridge's 1992 album Never Enough, on the EP for Rain Phoenix's band Papercranes, and on the score to Mission: Impossible III (2006), Mission: Impossible – Ghost Protocol (2011), and Star Trek Into Darkness (2013).
Mulroney attended Maury Elementary School and played cello in school and city youth orchestras, as well as acted in children's community theater. Mulroney has appeared in many movies, including as the male lead in My Best Friend's Wedding alongside Julia Roberts and Cameron Diaz.
Mulroney attended the Interlochen Arts Camp as a cellist. He is probably best known for his western roles as either an outlaw or gunman.
He occasionally reads stories on the WBEZ Chicago Public Radio.
In 2012, Mulroney played the love interest of Zooey Deschanel, Russell, in the Fox series New Girl.