William Keir Clark (May 30, 1910 – November 28, 2010) was a Canadian merchant and political figure in Prince Edward Island. After serving as mayor of Montague in 1941 and 1942, he represented 3rd Kings in the Legislative Assembly of Prince Edward Island from 1947 to 1959 and 4th Kings from 1966 to 1970 as a Liberal. In 1970, he sat as an Independent Liberal. He was defeated when he ran for reelection in 1959. In 2008 he became P.E.I.'s oldest living former politician, and he turned 100 in 2010. Clark died in Eldon, Prince Edward Island on November 28, 2010.