William Sydney Clack (31 March 1910-29 January 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.[1] He remained a councillor when 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.

In 1978 he married Jill Elizabeth Gould, who was also a member of Harrow Borough Council and the GLC.