An Army brat, Underwood grew up near bases around the globe. He studied acting at Carnegie-Mellon in Pittsburgh, Pennsylvania, before heading to New York hoping for a career in show business. He appeared in two episodes of NBC's "The Cosby Show": the first practically as an extra in an episode which really featured dancer Judith Jameson; then as boyfriend of Denise in a subsequent episode. The latter was the role that helped to bring him to the attention of Hollywood casting directors. Underwood first starred in a feature film in 1985, with "Krush Groove", the rap comedy in which he was the record producer trying to launch his own label, then played on the ABC daytime drama "One Life to Live" for a brief spell in 1986 as street kid Bobby Blue, before landing his first primetime series, playing Terry Corsaro, a paroled pickpocket, in the short-lived CBS venture "Downtown" (1986-87).

The exposure from "L.A. Law" helped Underwood break into TV-movies as well. His first was "The Cover Girl and the Cop" (NBC, 1989), but he is better recalled for playing Bob Richardson, a young man who has just moved to L.A. and finds himself in the midst of the Watts riots in "Heat Wave" (TNT, 1990). That same year, he played Civil Rights martyr James Chaney in NBC's "Murder in Mississippi", which tried, after so many outrageous fictions, to tell the true story of Chaney, Goodman and Schwerner, the trio who were murdered while working for integration in Philadelphia, Mississippi. With his brother Frank, Underwood formed a production company in the 90s and he also directed a short film, "The Second Coming" (1992), in which he played a Christ-like character. Underwood played a convicted criminal who meets his father (Louis Gossett Jr) for the first time behind bars, and then finds himself paroled alongside pop in "Father & Son: Dangerous Relations" (NBC, 1993). Also in 1993, he returned to the big screen was the sheriff in Mario Van Peebles' "Posse." In a less heroic mold was his convicted child rapist whom Sean Connery believes innocent in "Just Cause" (1995).

He was well-cast as Jackie Robinson in the 1996 HBO original film "Soul of the Game", which purported to tell the story of the bridge between the Negro Leagues and the Major Leagues. A string of supporting roles in major motion pictures followed, including the uban crime drama "Set It Off" (1996), the sci-fi conundrum "Gattaca" (1997), the big-budget disaster film "Deep Impact" (1998) and the military legal thriller "Rules of Engagement" (2000). He also kept a steady hand in television, joining the cast of ABC's short-lived police drama "High Incident" (1996-97), starring in the ensemble of the telepic adaptation of Alex Haley's familial novel "Mama Flora's Family" (1998) and took the lead in the ratings-challenged CBS medical drama "City of Angels" (2000).

In 2002 Underwood was cast in the high-profile ensemble of director Steven Soderbergh's muddled "Full Frontal" (2002), Soderbergh's attempt to return to indie sensibility after his mainstream Hollywood successes; The actor appeared opposite Julia Roberts, both in a movie-within-a-movie and as the actors playing their characters. In a sillier departure, Underwood joined the cast of "Malibu's Most Wanted" (2003), playing the campaign manager to a senator with a privileged, wannabe rapper son who acts and talks like he's straight out of the 'hood (Jamie Kennedy). That same year he began a series of recurring appearances on the hit HBO comedy "Sex and the City," playing Miranda's hunky physician neighbor and eventual paramour Robert Leeds. After being tapped by series creator Bill Cosby to provide the voice of the Cliff Huxtable-esque Dr. Arthur Bindlebeep in the animated Nick-At-Night series "Fatherhood" (2004 - ), Underwood returned to live action primetime network TV opposite Heather Locklear on the airport-set drama "LAX" (2004 - ), playing an ambitious airport executive overseeing the busy Los Angeles airport.