Gallagher made his film debut as a Fabian-like singer in "The Idolmaker" (1980) and followed with Randal Kleiser's menage-a-trois romance "Summer Lovers" (1981). He began to play ne'er-do-wells in "Dreamchild" (1985), cast as a reporter who hustles the elderly inspiration for Lewis Carroll's Alice and his breakthrough came as the arrogant, adulterous yuppie lawyer in Steven Soderbergh's "sex, lies and videotape" (1989). Gallagher displayed his aptitude for comedy as aggressive Hollywood executive Larry Levy in Robert Altman's "The Player" and a vacuous news anchor in Tim Robbins' "Bob Roberts" (both 1992) before returning to presumptuous lawyer mode with a supporting role in Harold Becker's thriller, "Malice" (1993). Altman featured him in "Short Cuts" (also 1993) as a helicopter pilot who extracts revenge on his cheating wife. The actor has remained a fixture in films, alternating between supporting roles in mainstream fare (e.g., the comatose yuppie in 1995's "While You Were Sleeping", the arrogant attorney in 1996's "Last Dance") and leads in smaller, independent features (especially Soderbergh's underrated 1995 noir thriller "The Underneath" and 1996's "To Gillian on Her 37th Birthday").

On the small screen, Gallagher has generally been associated with quality productions debuting as Karl Malden's medical student son in the TV-movie and short-lived spin-off series "Skag" (NBC, 1980). Other notable roles include a supporting role to Art Carney and James Cagney in "Terrible Joe Moran" (CBS, 1984), the accused murderer Leo Frank in "The Murder of Mary Phagan" (NBC, 1988), as one of the navy officers in Altman's TV remake of "The Caine Mutiny Court-Martial" (CBS, 1988), and as the prime suspect in a murder case who is romanced by a female undercover cop (Mare Winningham) in the superior "True Betrayal" (ABC, 1990). In 1993, Gallagher was featured in two short films made for the Showtime network's "Fallen Angels" anthology series: Tom Cruise's "The Frightening Frammis" and Steven Soderbergh's "The Quiet Room". He also scored as an ambitious advertising executive with a conscience in the drama "White Mile" (HBO, 1994). More recently, he co-starred as a fictitious passenger on the doomed liner in the CBS miniseries "Titanic" (1996), an overworked FBI agent investigating a terrorist incident in "Path to Paradise: The Untold Story of the World Trade Center Bombing" (HBO, 1997) and was featured in the NBC remake of "Brave New World" (1998).

In 1998, Gallagher returned to series TV as a businessman in the throes of a divorce in the short-lived ABC sitcom "The Secret Lives of Men". That series' failure to find an audience was certainly levened by the success of Gallagher's next project, director Sam Mendes' powerful and affecting comedy-drama "American Beauty" (1999), in which the actor was particularly effective in a prominent supporting role as Annette Bening's employer and idol (and eventual lover), the smarmy realtor Buddy "The King" Kane. His next project, a modern remake of the cult horror classic "The House on Haunted Hill" (1999), was less impressive, but that same year he earned critical praise for his turn in the TV movie "Brotherhood of Murder" for his role as a volatile white supremist--he would reunite with co-star Steven Baldwin in 2001 for the B-grade thriller "Protection." Gallagher next appeared in several little-seen or forgettable features, indies and telepics--including a turn as an aging choreographer in director Nicholas Hytner's ill-received dancer drama "Center Stage" (2000)--including before resurfacing in Showtime's major television miniseries, an adaptation of novelist Anne Rice's "Feast of All Saints" (2001) as Philippe Ferronaire, and as scheming my corporate CEO Chuck Cedar opposite sweet-natured small town guy Longfellow Deeds (Adam Sandler) in "Mr. Deeds," a 2002 remake of "Mr. Deeds Goes to Town." Gallagher next appeared as Mandy Moore's romantically challenged father in the teen romance "How to Deal" (2003) before making another foray into series television as Sandy Cohren, the patriarch of a Newport Beach clan, on the WB youth drama "The O.C." (2003 - ).