He entered the United States Army in July 1941 during World War II.

He served in the United States Army for two years and in the European Theater of Operations for two and a half years with the Twenty-ninth Infantry as platoon leader, company commander, and operations officer.

Jennings owned an automobile and farm implement business in Marion, Virginia, from 1946 until his death.

He was elected as a Democrat to the Eighty-fourth Congress and to the five succeeding Congresses (January 3, 1955 – January 3, 1967), during which time he was a signatory to the 1956 Southern Manifesto that opposed the desegregation of public schools ordered by the Supreme Court in Brown v. Board of Education.

In 1966, journalist Drew Pearson reported that Jennings was one of a group of four Congressmen who had received the "Statesman of the Republic" award from Liberty Lobby for their "right-wing activities".