He was first recognized for his performance of teen rebel Zack Dell in the film Camp Nowhere (1994). Keegan was cast in a minor role in the 1996 film Independence Day. He also played a love interest for Stephanie Tanner in the television series Full House for an episode. He had guest parts on numerous television shows before being cast on the TV drama Party of Five (1994). That same year, he landed another recurring role on 7th Heaven (1996), where he played a single teenaged father in love with Jessica Biel's character Mary. Keegan was then cast in two modern-day Shakespearean film adaptations: as the antagonist of Heath Ledger in the 1999 comedy 10 Things I Hate About You (adapted from The Taming of the Shrew) and as Mekhi Phifer's best friend in O, adapted from Othello.