William Mansfield Clark (17 August 1884 – 19 January 1964) was an American chemist and professor at the Johns Hopkins University.

Clark was born in Tivoli, New York, in a clergy family and studied at Hotchkiss School and Williams College before entering Johns Hopkins University, where he received a PhD in chemistry under H.N.

He then worked on dairy bacteriology in the US department of agriculture followed by studies on oxidation reduction of dyes and metalloporphyrins at the Hygiene Laboratory, which he headed from 1920.

He joined the Johns Hopkins Medical School as a professor of physiological chemistry in 1927.

He was elected to the National Academy of Sciences in 1928 and to the American Philosophical Society in 1939.