Corporal Mainhart was a member of Company I, 3rd Battalion, 31st Infantry Regiment, 7th Infantry Division. He was Killed in Action while fighting the enemy in North Korea on November 30, 1950. His remains were not recovered. In September and October 2004, personnel from the Joint POW/MIA Accounting Command (now DPAA), conducted the 36th Joint Recovery Operation with the Korean People's Army in the vicinity of the Chosin River. During the mission, a witness statement reported that remains believed to be American had been found and reburied. Recovery Team 2 found a site that contained material evidence and possible remains of at least five individuals. DNA analysis and circumstantial evidence were used in the identification of his remains. He was accounted for in 2016. His name remains permanently inscribed on the Courts of the Missing at the Honolulu Memorial.