After the war ended in 1918, his family went back to Komotau (now Chomutov), the hometown of his father in Bohemia, which is now part of the Czech Republic.

He served as a non-commissioned master radioman (Oberfunkmeister) in the campaigns in France and Russia, although his assignments being primarily in the rear areas, he did not see actual combat.

When the German 16th Army capitulated in the Courland Pocket (Kurland-Kessel), he was held by the Soviets as a prisoner of war from June 1945 to December 1947.

His ability to manage people and his desire to cooperate with his Soviet superiors led to a rapid rise in the prisoner leadership.

During this early party work he met Waldemar Verner who was the first district Secretary of the SED in Stralsund who was also later Ehm's predecessor as Chief of the People's Navy (Volksmarine).

Ehm was different from other early East German military leaders, in that he lacked their long history as communists or socialists since the Weimar Republic.

Ehm was promoted to Vizeadmiral in March 1964, and commanded the People's Navy until his retirement on 30 November 1987, with the exception of August 1961 to February 1963, when he attended the Soviet Naval Academy, graduating with distinction.