Lethin joined the United States Marine Corps in 1980 as a second lieutenant. He was assigned to the second battalion, third Marines in Hawaii and was later deployed to Desert Storm. For six years, he planned military training exercises in Egypt, Jordan and Kenya. Later, he participated in combat operations in Afghanistan in 2001, and Iraq from 2003 to 2004.
After retiring in 2008, he continued to serve, joining the Trojan Family as he took on his current role at USC.
