Born into a wealthy family in St. Louis, Missouri to Judge Thomas C. Hennings Sr, his father was an influential member of the Jefferson Club, an organization dedicated to overthrowing the bossism of the city's Democratic Political Machine. Hennings Jr. attended Soldan High School, and displayed talents in athletics, going on to run track and field at Cornell University. After graduating from Cornell in 1924, he finished his education at the law school of Washington University in St. Louis in 1926. He was admitted to the bar in 1926 and commenced practice in St. Louis, and served as assistant circuit attorney for that city from 1929 to 1934. He served as a colonel on the Governor's staff from 1932 to 1936 and was a lecturer on criminal jurisprudence at the Benton College of Law in St. Louis from 1934 to 1938.
Hennings was elected to the Seventy-fourth, Seventy-fifth, and Seventy-sixth Congresses and served from January 3, 1935, to December 31, 1940. He was the first Democrat in 22 years to represent Missouri's 11th congressional district. Hennings enjoyed support from St. Louis' growing African American population. Hennings regularly hired African Americans to his offices in Washington D.C. and St. Louis. During the Second New Deal, he worked towards establishing an African American branch of the Civilian Conservation Corps at Poplar Bluff, Missouri. Within the New Deal'sFederal Emergency Relief Act, Hennings advocated for a "Negro Federal Employment Office" with all black staff. Also during his time in the House, Hennings sponsored the perennial failed anti-lynching bill, His work secured him the endorsements of two of St. Louis' black newspapers, the Argus and the American and the city's NAACP chapter. Hennings resigned in 1940 to become a candidate for circuit attorney of St. Louis.
Time out of Office
Hennings was circuit attorney from 1941 to 1944, taking leave of absence in September, 1941 to volunteer in active duty in the United States Naval Reserve. Serving in the Pacific and Caribbean as a lieutenant commander for three years, he was discharged from active duty due to physical disability incurred in the line of duty. After which, he resumed the practice of law in the St. Louis firm of Green, Hennings, Henry and Evans.
Senate
He was elected as a Democrat to the U.S. Senate in 1950 over Republican incumbent and former governor Forrest Donnell in the only senate election that year where Democrats took a seat from Republicans, was reelected in 1956, and served from January 3, 1951, until his death from abdominal cancer in Washington, D.C. in 1960. Hennings did not sign the 1956 Southern Manifesto, and voted in favor of the Civil Rights Acts of 1957 and 1960. While in the Senate, he was chairman of the Committee on Rules and Administration.