Jennings graduated in June 1941 with a Doctor of Medicine degree and accepted a commission as a 1st lieutenant in the Medical Corps Reserve, but remained at the university to complete a one-year internship.

On July 1, 1942, he was called to active duty and attended a one-month refresher course at the Medical Field Service School, Carlisle Barracks, Pennsylvania.

In August, he was assigned to McCornack General Hospital in Pasadena, California to work in plastic and reconstructive surgery on casualties of World War II.

On August 1, 1956, he was promoted to permanent lieutenant colonel and concurrently assigned to Walter Reed Army General Hospital, Washington, D.C.

From July 1961 Jennings served in Germany as a staff surgeon, V Corps, in Frankfurt for a year and as commander of the 130th Station Hospital in Heidelberg starting in August 1962.

On his return to the United States in August, Colonel Jennings assumed command of Martin Army Hospital, Ft. Benning, Georgia, and concurrently served as post surgeon.

On February 1, 1969, he assumed command of the 44th Medical Brigade at Long Binh, with additional duty as surgeon, Headquarters, United States Army, Vietnam.