In 1930 Randolph lost a bid for election to the U.S. House of Representatives, but he won the seat in 1932 and was re-elected six times, serving from March 4, 1933, to January 3, 1947.

He had a longtime association with Southeastern University in Washington, D.C., serving as a professor of public speaking there from 1935–1953, and dean of the School of Business Administration from 1952 to 1958.

[11] Randolph was best known for sponsoring eleven times an amendment to the Constitution that would grant citizens aged between 18 and 21 the right to vote.

The legislation gave the CAA the power to regulate airline fares and to determine the routes that air carriers would serve.

In 1942 Randolph proposed a Synthetic Liquid Fuels Act, which would fund the transformation of coal and its products into other useful forms of energy.

To promote the viability of synthetic fuels, in November 1943 Randolph and a professional pilot flew in an aircraft powered by gasoline derived from coal.

The small, single-engine airplane flew from Morgantown, West Virginia to National Airport in Washington, D.C. Aided by Interior Secretary Harold Ickes and Senator Joseph C. O'Mahoney, the Synthetic Liquid Fuels Act was approved on April 5, 1944.

He introduced legislation to establish a Department of Peace in 1946 with the goal of strengthening America's capacity to resolve and manage international conflicts by both military and nonmilitary means.

In the 1970s and 1980s he joined senators Mark Hatfield and Spark Matsunaga and Congressman Dan Glickman in efforts to create a national institution dedicated to peace.

After he had announced his retirement from Congress in 1984, Randolph played a key role in the passage and enactment of the United States Institute of Peace Act.

The Jennings Randolph Program, which awards fellowships to enable outstanding scholars, policymakers, journalists, and other professionals from around the world to conduct research at the U.S. Institute of Peace, has been named in his honor.