McMahon began a practice in Norwalk and later served as a judge on that town's city court,[1] appointed to the position by Connecticut Governor Wilbur L.

Although he lost, he "received wide public recognition and a reputation as a courageous and honest upholder of justice, both of which would further his political ambitions," according to a biography accompanying the introduction to his papers, held by the Georgetown University Library.

[2] On July 16, 1945, an atomic bomb was successfully detonated at Alamogordo, New Mexico, after which Senator Brien McMahon of Connecticut called it "the most important thing in history since the birth of Jesus Christ."

In late 1945, McMahon was appointed Chairman of the Senate Special Committee on Atomic Energy, which explored legislative alternatives to the War Department sponsored May-Johnson bill.

McMahon lacked knowledge about atomic energy, but saw the chairmanship as a means to assert himself as a new Senator, especially as the May-Johnson bill underwent increased attack from scientists and later lost support of the Truman White House.

[4] Moreover, during the next several months there was an intense debate within the U.S. government, military, and scientific communities regarding whether to proceed with development of the far more powerful hydrogen bomb, then known as "the Super".

Beginning in January 1952, McMahon was mentioned as a possible candidate in the 1952 Democratic Party presidential primaries, but he vacillated over whether he was actually running or not.

[1] From his sickbed, he sent a message to the Democratic state convention in Hartford, Connecticut saying that if elected president, he would tell the Atomic Energy Commission to manufacture thousands of hydrogen bombs.

[6] By the time of the 1952 Democratic National Convention in July, he was too weak to be considered an actual candidate, but the delegation from Connecticut initially cast their 16 votes for him as a symbolic gesture.

[1] Brien McMahon served in the United States Senate until his death at Georgetown University Hospital in Washington, D.C., on July 28, 1952, at age 48.

A commemorative stamp honoring Brien McMahon and his role in opening the way to peaceful uses of atomic energy was issued by the United States on July 28, 1962, at Norwalk, Connecticut.