Lewis Hershey was born on September 12, 1893, in Steuben County, Indiana. As a youngster, Lewis attended the local public schools. After his high school graduation, Lewis trained as a teacher at Tri-State College, now known as Trine University. After earning a bachelor of science in 1912 and a bachelor of arts in 1914, he taught at local elementary schools and served as a school principal in Flint, Indiana.
In 1911, Lewis enlisted in the Indiana National Guard. Almost immediately, his guard unit was called to active duty on the Mexican border, where they served until December, 1916. That year he was commissioned as a second lieutenant. His unit was again called to federal service during World War I, when Lewis was deployed to France as part of the American Expeditionary Force. After the war ended, Lewis continued his education at both the Command and General Staff College and the Army War College, graduating in 1933. Then Lewis taught military science at Ohio State University.
In 1936, Lewis was assigned to the General Staff in Washington, D.C. In 1940 President Franklin Roosevelt promoted the former teacher to brigadier general, and in 1941 President Roosevelt designated him Director of the Selective Service. Lewis was the longest-serving director in the history of the Selective Service System, and holding the position until 1970. His years of service spanned World War II, the Korean War, and the Viet Nam War. General Hershey was one of only three generals in the history of the United States Army to have served as a general during three major conflicts. Despite this distinction, his service during the Viet Nam War generated a great deal of controversy.