William Mapother (sometimes credited as William R. Mapother) is an American actor and former teacher best known for his recurring role as Ethan Rom on the television drama series Lost.
He has appeared in numerous other television shows, including CSI: Crime Scene Investigation, Law & Order: Special Victims Unit, Touched by an Angel, CSI: Miami, NCIS, Threshold, Crossing Jordan, and Robot Chicken.
The cousin of Tom Cruise, Mapother has worked as an assistant on four of Cruise's films: Cocktail (1988), Rain Man (1988), Born on the Fourth of July (1989) and Mission: Impossible II (2000).
He has acted in the Cruise films Born on the Fourth of July, Mission: Impossible II, Vanilla Sky (2001), and Minority Report (2002).
Other films Mapother has acted in include Without Limits (1998), Magnolia (1999), Swordfish (2001), Self Storage (2002), Suspect Zero (2004), The Grudge (2004), Lords of Dogtown (2005), The Zodiac (2005), and World Trade Center (2006).
He also had minor roles in the films Almost Famous (2000) and Mean Creek (2004), but his scenes were deleted from the final cut of both movies.
Mapother was a school teacher in East Los Angeles for three years before becoming an actor.