William Sydney Clack (31 March 1910-1984) was a Conservative Party local politician. Born in Fulham, he was the son of a baker. He moved to Pinner in the Borough of Harrow, Middlesex, and in 1955 was elected to fill a casual vacancy on the borough council. He remained a councillor when the the municipal borough became the London Borough of Harrow in 1965, later becoming an alderman and Mayor of Harrow for 1969-70. He sat as a member of the Greater London Council representing Harrow 1970–1973 and Harrow Central 1973–1981 Committee Chair Planning North Area 24 May 1977.