Dorner was a United States Navy Reserve officer, commissioned in 2002. He commanded a security unit at the Naval Air Station Fallon (Fallon, Nevada), served with a Mobile Inshore Undersea Warfare Unit from June 2004 to February 2006, and was deployed to Bahrain with Coastal Riverine Group Two from November 2006 to April 2007.[17] He was honorably discharged from the Navy Reserve as a lieutenant on February 1, 2013.[18]

In 2002, while a student at Undergraduate Pilot Training at Vance Air Force Base (Enid, Oklahoma), Dorner and a classmate found a bag containing nearly US$8,000 (equivalent to about $12,100 in 2021) that belonged to the nearby Enid Korean Church of Grace. The two handed the money to the police. When asked their motive, Dorner replied that, "The military stresses integrity. ... There was a couple of thousand dollars, and if people are willing to give that to a church, it must be pretty important to them." Dorner also stated his mother taught him honesty and integrity.[19] During his time as a reservist, Dorner received a Navy Rifle Marksmanship Ribbon and a Navy Pistol Shot Ribbon with expert device.[20][21]