William Flanagan may refer to:
William Flanagan (composer) (1923–1969), American composer
William Flanagan (American football) (1901–1975), professional football player
William Flanagan (politician) (1871–1944), British Member of Parliament for Manchester Clayton, 1931–1935
William J. Flanagan Jr. (born 1943), U.S. Navy admiral
William A. Flanagan (born 1980), former mayor of Fall River, Massachusetts