William or Bill Leonard may refer to:

• William Leonard (English politician) (fl. 1597), MP for Dover
• William Leonard (Scottish politician) (1887–1969), MP for Glasgow St. Rollox, 1931–1950
• J. William Leonard, director of the Information Security Oversight Office
• Bill Leonard (politician) (born 1947), American politician in California
• Billy Leonard (born 1955), Irish republican politician

• William Leonard (Medal of Honor), Medal of Honor recipient during the Plains Indian Wars
• William E. Leonard (1836–1891), Medal of Honor recipient in the American Civil War
• William F. Leonard (1913–1985), recipient of the Medal of Honor
• William N. Leonard (1916–2005), American World War II flying ace and United States Navy admiral

• William Andrew Leonard (1848–1930), American author and prelate of the Episcopal Church
• William Ellery Leonard (1876–1944), American poet
• William J. Leonard (1927–2006), American football player
• Bill Leonard (runner) (born 1923), American miler, 1948 1500 m All-American for the Notre Dame Fighting Irish track and field team
• William Robert Leonard or Bobby Leonard (1932–2021), American basketball player and coach
• Bill Leonard (journalist) (1916–1994), American broadcast journalist
• Bill J. Leonard (born 1946), American historian of religion