William Mack (February 29, 1828 – December 11, 1897) was an Ontario businessman and political figure. He represented Cornwall from 1879 to 1883, Cornwall and Stormont from 1886 to 1890 and Stormont from 1890 to 1894 in the Legislative Assembly of Ontario as a Liberal member. His daughter Margaret married Alexander McCracken, who was a mayor of Cornwall, and his son William R. later served as sheriff for the United Counties.