William Mackay or MacKay may refer to:

• William Mackay (artist) (1876–1939), American artist
• William Andrew MacKay (1929–2013), Canadian lawyer, judge and university president
• William Paton Mackay (1839–1885), Scottish doctor, Presbyterian minister and hymn writer
• William MacKay (politician) (1847–1915), physician and political figure in Nova Scotia
• William Alexander Mackay (1860–1927), Scottish doctor who co-founded Spain's oldest football club, Recreativo de Huelva
• William John MacKay, Ontario politician

• William McKay (disambiguation)
• William McKee (disambiguation)
• William McKie (disambiguation)