Murtha left Washington and Jefferson College in 1952 to join the Marine Corps and was awarded the American Spirit Honor Medal for displaying outstanding leadership qualities during training. Murtha rose through the ranks to become a drill instructor at Parris Island and was selected for Officer Candidate School at Quantico, Virginia. Murtha was then was assigned to the Second Marine Division, Camp Lejeune, North Carolina.
Murtha remained in the Marine Corps Reserves, and ran a small business, Johnstown Minute Car Wash. Murtha attended the University of Pittsburgh on the G.I. Bill, and received a degree in economics. Murtha later took graduate courses from the Indiana University of Pennsylvania. Murtha married his wife Joyce on 10 June 1955 — they have three children and live in Johnstown.
In 1959, Murtha, then a captain, took command of the 34th Special Infantry Company, Marine Corps Reserves, in Johnstown. He remained in the Reserves after his discharge from active duty until he volunteered for service in the Vietnam War, serving from 1966 to 1967, serving as a battalion staff officer (S-2 Intelligence Section), receiving the Bronze Star with Valor device, two Purple Hearts and the Vietnamese Cross of Gallantry. He retired from the Reserves as a colonel in 1990, receiving the Navy Distinguished Service Medal.