Keegan was born Andrew Keegan Heying in Los Angeles, California, the older son of Lana (nee Ocampo), a hairdresser, and Larry Heying, a voice-over actor. Keegan's mother is Colombian and his father is from Nebraska. Keegan has a younger brother, Casey (born June 12, 1980), who is also an actor.

He is perhaps best known for his roles in films such as 10 Things I Hate about You (1999), The Broken Hearts Club (2000) and O (2001). He had recurring roles on several television series, including Full House, Party of Five, 7th Heaven, Related, Step By Step, Sabrina The Teenage Witch and The Amanda Show, and was a regular in the final season of Thunder Alley. He also starred as Zack Dell in the movie Camp Nowhere (1994) and had a small role in Independence Day (1996) He also played Dan Foster in "The Skateboard Kid 2" (1995). In 2009 Keegan made his stage debut as Rigby in Erik Patterson's award winning play He Asked For It.

Most recently he has appeared on the hit television show, House, on the Season One episode; Three Stories (2005).