William Marchant was a London art dealer, the son of a Bristol iron-founder. He married Cecily Gertrude Marchant (d. 1955) in 1900.
Marchant was educated in France after the death of his parents and joined the Paris firm of Boussod, Valadon & Cie, art dealers, at the age of seventeen. In 1890, he joined the firm's London branch, known as the Goupil Gallery.
The gallery ran exhibitions of French and British art and twice yearly Salons as well as representing individual artists including Whistler. In 1898, Marchant became gallery manager.
Two years later, he took over the London business although it took a court case brought by the Paris firm in 1907 before he was able to keep the Goupil name. Marchant continued to run the Goupil Gallery until his death in 1925 at the age of 57, when his wife took over the business. The Goupil Gallery was destroyed by bombing in 1941 during the Second World War.
Papers of the Marchant family and the Goupil Gallery, Hyman Kreitman Research Centre, Tate Britain; Houfe, Simon, 'David Croal Thomson, Whistler's Aide-de-camp,' Apollo, vol. 99, 1984, pp. 112-19.