William was the son of William Grant and Dorothea Dalrymple. William the elder was a lawyer, owner of an estate called Congalton, in Scotland. William the younger had a sister Elizabeth (d. 1899), who married Charles Pelham-Clinton (1813-1894), second son of the 4th Duke of Newcastle-under Lyme. William Grant the elder may be best remembered as the subject of Gilbert Stuart's full length portrait, The Skater, now in the NGA (1950.18.1).
[Compiled from sources and references recorded on CMS]
Miles, Ellen G. American Paintings of the Eighteenth Century. The Collections of the National Gallery of Art Systematic Catalogue. Washington, D.C., 1995: 163-168 [on Stuart's portrait of William Grant]