In September 1924, he was appointed a second lieutenant in the United States Army Reserve, serving as an ordnance officer.

After the war, as a civilian, Slezak helped to establish the machine tool division of the Army-Navy Munitions Board.

In 1953, President of the United States Dwight D. Eisenhower nominated Slezak as Assistant Secretary of the Army (Materiel) and he subsequently held this post from May 4, 1953 until February 7, 1954.

Eisenhower then nominated him as United States Under Secretary of the Army and he subsequently held this post from February 1954 until January 1955.

From 1957 until his retirement in 1978, Slezak served as chairman of the Reserve Forces Policy Board of the United States Department of Defense.