Mein served as the United States Ambassador to Guatemala (1965-1968) during the Guatemalan Civil War. He was shot by rebels belonging to the Fuerzas Armadas Rebeldes (FAR. "Rebel Armed Forces") one block from the United States consulate on Avenida Reforma in Guatemala City on August 28, 1968.
United States officials believed that FAR intended to kidnap him in order to negotiate an exchange, but instead they shot him when he attempted to escape. He is buried at Rock Creek Cemetery, in Washington, District of Columbia.