William Mercer may refer to:
- William Mercer (poet) (c. 1605–c. 1675), Scottish poet and army officer
- William Thomas Mercer (1821–1879), British Colonial Secretary in Hong Kong, 1859–1868
- William W. Mercer, American politician from Montana and former United States Attorney
- William (Rosko) Mercer (1927–2000), known as Rosko, American news announcer and disc jockey
- Bill Mercer (born 1926), American sportscaster
- William Mercer (rugby league) (born 1906), rugby league footballer of the 1930s for England and St Helens RLFC
- William Mercer (cricketer) (1922–1989), English cricketer
- William Mercer (Australian politician) (1796–1871), British Army officer, landowner, pastoralist, and politician in colonial New South Wales
- Willie Mercer (1874–1932), Scottish footballer