May was born on Beaver Creek, near Prestonsburg in Floyd County, Kentucky, on June 24, 1875. On June 25, 1898, he and his twin brother William H. May graduated from Southern Normal University Law School in Huntingdon, Tennessee (later named Union University, Jackson, Tennessee), and was admitted to the bar the same year, commencing his law practice in Prestonsburg. May and his brother formed the law firm of May & May which was not dissolved until the death of his brother on February 20, 1921. May was county attorney of Floyd County, Kentucky, 1901–1909; special judge of the circuit court of Johnson and Martin Counties in 1925 and 1926. During this time, May also engaged in Democratic Party politics, agricultural pursuits, coal mining and banking.
May was elected as a New Deal Democrat to the Seventy-second Congress and to seven succeeding Congresses (March 4, 1931 – January 3, 1947). He was Chairman of the powerful Committee on Military Affairs during the Seventy-sixth through Seventy-ninth Congresses, and a consistent supporter of the Franklin D. Roosevelt administration. During World War II, May became involved with Murray and Henry Garsson, New York businessmen who sought lucrative munitions contracts then being awarded by the U.S. Government.
Historian Clay Blair claimed that May was responsible for a major release of highly confidential military information during World War II, known as the May Incident. In that incident, U.S. submarines had been conducting a successful undersea war against Japanese shipping during World War II, frequently escaping Japanese anti-submarine depth charge attacks. However, the deficiencies of Japanese depth-charge tactics were revealed in a press conference held in June 1943 by Congressman May on his return from a war zone junket. At this press conference, May revealed the highly sensitive fact that American submarines had a high survival rate because Japanese depth charges were typically fuzed to explode at too shallow a depth. Various press associations sent this leaked news story over their wires and many newspapers (including one in Honolulu, Hawaii), published it.
It was subsequently discovered that Japanese naval antisubmarine forces were adjusting their depth charges to explode at a deeper depth. Vice Admiral Charles A. Lockwood, commander of the U.S. submarine fleet in the Pacific, later estimated that May's security breach cost the United States Navy as many as ten submarines and 800 crewmen killed in action, stating "I hear Congressman May said the (Japanese) depth charges are not set deep enough. He would be pleased to know (they) set them deeper now." A report from the U.S. Navy's Pacific Submarine Fleet determined that Japanese anti-submarine warfare (ASW) forces failed to uncover the maximum test depth ability of U.S. fleet submarines during the war. However, the report made no finding as to whether Japanese ASW forces altered their depth charge attacks to deeper settings as a consequence of May's revelation to the press.
Sometime shortly before or during the U.S. entry into World War II, May became involved with Murray Garsson and Henry Garsson, New York businessmen with no prior arms manufacturing experience who sought lucrative munitions contracts then being awarded by the U.S. Government. May was known to frequently telephone army ordnance and other government officials on the Garssons' behalf to award war contracts, obtain draft deferments, and secure other favors for the Garssons and their friends. So numerous were these interventions that one ordnance official referred to them as "blitz calls." After the war, a Senate investigating committee reviewing the Garsson's munitions business discovered evidence that May had received substantial cash payments and other inducements from the Garssons.
Following news reports of irregularities concerning his conduct in office, May was an unsuccessful candidate for reelection in 1946 to the Eightieth Congress. The bribery scandal was intensified by testimony of excessive profit-taking in the Garsson munition business, and that the Garsson factory produced 4.2-inch mortar shells with defective fuzes, resulting in premature detonation and the deaths of 38 American soldiers. After less than two hours of deliberation, May was convicted by a federal jury on July 3, 1947, on charges of accepting bribes to use his position as Chairman of the Military Affairs Committee to secure munitions contracts during the Second World War. Murray and Henry Garsson also received prison terms. After protracted efforts to avoid incarceration, May subsequently served nine months in federal prison.
However, he continued to retain influence in Democratic party politics, and President Truman decided to grant May a full pardon in 1952. Unable to revive his political career, he returned home to practice law until his death.
May died in Prestonsburg, Kentucky on September 6, 1959, and is buried in Mayo Cemetery.
The lodge at Jenny Wiley State Resort Park in Prestonsburg, Kentucky, was named after May by Governor Bert T. Combs.