Hull was born Josephine Sherwood in Newtonville, Massachusetts, and attended Radcliffe College and The New England Conservatory of Music. She made her stage debut in stock in 1905, and spent five years as a chorus girl and touring stock before she married Shelley Hull in 1910. Her husband died in 1919, and in 1923 Hull returned to show business under the name Josephine Hull.
Hull was a stage success in Craig's Wife (1926), and in Daisy Mayme (1926), a role which was written especially for her. Through the 1920s she continued working in the theater, and in the 1930s had three Broadway hits in You Can't Take It With You (1936), Arsenic and Old Lace (1941), and Harvey (1944).
Hull made a total of five films. She brought her two best stage roles to film in Arsenic and Old Lace (1944) playing a homicidal aunt, and in Harvey (1950) as the batty sister of a man whose friend is an invisible rabbit, for which she won the 1950 Oscar as Best Supporting Actress.
Hull made one more film, The Lady from Texas (1951), and appeared in a TV version of Arsenic and Old Lace in 1949, before retiring. She died of a cerebral hemorrhage in 1957.