He was a second lieutenant in the United States Army Reserve from 1938 to 1941, while serving in private practice in Mapleton, Iowa from 1940 to 1941.
After the United States entered World War II, he served in North Africa and Italy with the 34th Infantry Division of the United States Army from 1941 to 1946, rising to the rank of captain.
[2] Before his appointment, while serving as chairman of the Polk County Republican Central Committee, Stephenson had been an early and active supporter of Eisenhower's candidacy.
[3] Stephenson was nominated by President Dwight D. Eisenhower on May 16, 1960, to a seat on the United States District Court for the Southern District of Iowa vacated by Judge Edwin Richley Hicklin.
[2] Stephenson was nominated by President Richard Nixon on June 1, 1971, to a seat on the United States Court of Appeals for the Eighth Circuit vacated by Judge Martin Donald Van Oosterhout.