William Leon St. Onge (October 9, 1914 – May 1, 1970) was a United States Representative from Connecticut.

He then attended the University of Connecticut School of Law at Hartford in 1948, and was admitted to the bar in 1948.

He was a member of board of education of Putnam in 1939–1941, and served in the State House of Representatives in 1941–1942.

He was chairman and executive director of the redevelopment agency of Putnam, 1956–1958, and was mayor of the city in 1961–1962.

In 1962 he was elected as a Democrat to the Eighty-eighth and to the three succeeding Congresses, serving from January 3, 1963, until his death from a heart attack in Groton, Connecticut in 1970.