The product of a wealthy Hudson Valley family that traced its ancestry back to the colonial period, Wadsworth was born in Groveland, New York, on June 12, 1905. His relatives included John Hay, Lincoln's private secretary who later served as secretary of state, and his father was a U.S. senator. He attended Fay School and St. Mark's School. Following his graduation from Yale University in 1927, Wadsworth returned to his family estate to manage his own dairy farm. Wadsworth won election as a Republican to the New York Assembly in 1930 and served until 1941 (he did not run for reelection in 1940). Upon leaving the legislature, he went to work for the Curtis-Wright Corporation in Buffalo, New York, where he rose to the position of assistant manager of industrial relations and earned a reputation as a skillful labor negotiator. In 1945, he was appointed director of the public service division of the War Assets Administration. Between 1946 and 1948, he headed the governmental affairs department of the Air Transport Association.
In 1948, Wadsworth became a special assistant to Paul G. Hoffman, head of the Economic Cooperation Administration. In that capacity, he helped line up congressional support for the Marshall Plan. In June 1950, he became administrator of the civil defense office of the National Security Resources Board. The following year, he received an appointment as deputy administrator of the Civil Defense Administration.
At the urging of Henry Cabot Lodge, the incoming administration's designated ambassador to the United Nations and a Yale classmate of Wadsworth's, President Dwight D. Eisenhower appointed Wadsworth deputy representative to the UN in 1953. He sat on the Economic and Social Council and was an alternate representative to the Technical Assistance Conference. As a representative, Wadsworth had no policy-making role but served as spokesman for the administration on many sensitive issues. He opposed the blanket admission of new members, which could lead to the entrance of Communist China to the international organization, and barred any negotiations on Korean unification until North Korea recognized the "authority of the United Nations in repelling the Communist invasion of South Korea." Wadsworth also urged the Security Council in 1955 to censure Israel for attacking the Gaza Strip. He advised the Arab nations to recognize the right of Israel to exist and recommended that they resettle Palestinian refugees from Israel permanently and regard them "not as temporary residents but as fellow citizens and co-sharers of the Near East's future."
Wadsworth's primary responsibilities involved disarmament and atomic energy. As deputy representative to the UN Disarmament Commission, Wadsworth took part in negotiations for arms reduction. Speaking before the General Assembly in October 1954, he voiced the Eisenhower administration's policy that disarmament must be accompanied by an effective inspection system. "We cannot stop an arms race unless all the racers stop running," he said, "and we cannot know whether all the racers have stopped running if one of them insists on running on a concealed track. For the free world to stop arming while the Soviet Union keeps on increasing its strength would be an invitation to the very war we seek to avoid."
As U.S. representative to the Disarmament Commission's Subcommittee on Atomic Control, Wadsworth reiterated the administration's demand for an end to secrecy in nuclear development and ready access to atomic installations for inspection purposes. In May 1955, he cautioned that Soviet proposals for a self-inspection system "still appear to fall short of the minimum safety requirements." He responded unequivocally to Soviet foreign minister Andrei Y. Vishinsky's demand in October 1954 that the UN Security Council be given veto powers in any disarmament formula. Wadsworth believed that Vishinsky's proposal would make the Soviets the supreme arbiter in deciding which disarmament violations were to be punished.
Wadsworth gradually achieved a reputation as the administration's "chief troubleshooter." His abilities as a negotiator earned him the praise of many of his colleagues in the UN. During 1956, he handled the extremely delicate negotiations leading to the formulation of the Statute of the International Atomic Agency, establishing a 70-nation organization to further peaceful use of atomic energy. In October 1956, Dr. João Carlos Muniz of Brazil, chairman of the council that elaborated the fine points of the statute, called it a "monument to Wadsworth."
In February 1958, Secretary of State John Foster Dulles appointed Wadsworth to replace Harold Stassen As chief U.S. disarmament negotiator. Wadsworth represented the United States at the disarmament conference in Geneva, Switzerland. Although the United States and the Soviet Union continued to disagree on the issue of inspections (the United States pressed for on-site inspections, while the Soviets would agree only to "seismic" inspections from outside national borders), they reached agreement on 17 of 21 articles that became part of the treaty signed in 1963.
Wadsworth later replaced Henry Cabot Lodge in September 1960 as permanent U.S. representative to the UN. He proved to be a popular diplomat, known for his informal manner and ready laugh as well as his diplomatic expertise. The Russians who worked with him found him "serious, not a cold warrior." Wadsworth left his UN post in 1961. He served as a government consultant during the 1960s, and President Lyndon B. Johnson Appointed him to he Federal Communications Commission, on which he sat from 1965 to 1970. Wadsworth retired to his family farm in the Genesee Valley of upstate New York. He died in Rochester, New York, on March 13, 1984.