Phillips is best known for his portrayal of ill-fated rock 'n' roller Ritchie Valens in La Bamba (1987). He also appeared as a troubled gangster in Stand and Deliver (1988) and shared the screen with fellow teen idols Kiefer Sutherland and Emilio Estevez in Young Guns (1988) and Young Guns II (1990).
Later in his career, Phillips made the transition to Broadway, earning Tony and Drama Desk nods for his portrayal of the king in "The King and I" in 1996. Television credits include "24," "Law & Order: Special Victims Unit" and "Numb3rs."