Stethem was born in Waterbury, Connecticut, but grew up in Virginia Beach, Virginia, and Waldorf, Maryland. He was one of four children. His father, Richard Stethem, retired from the Navy as a Senior Chief after 20 years, continuing to work for it as a civilian afterwards. His mother, Patricia, served in the Navy before raising her family, continuing to serve in the U.S. Court of Appeals for the Armed Forces afterwards. His brother, Chief Boatswain’s Mate Kenneth Stethem was a Navy SEAL and brother Diver First Class Patrick Stethem served in Underwater Construction Team One for 10 years – the same unit in which Robert served. Stethem had one sister, Sheryl Sierralta.