William was the son of Henry Purdon Sinclaire, Sr., (1834-1902) and his first wife, Frances Augusta (Oakes) Sinclaire (1833-1872).
Henry was born in Belfast, Ireland. He was living in Corning, Steuben County, New York, in the 1880 U.S. Census, and his occupation was "glass manufacturer." His son, William, was "at school" in 1880.
William was educated in the Corning Free Academy and the Phillips Exeter Academy in New Hampshire. His entire business career was associated with the Corning Glass Works, of which he was a director as well as secretary-treasurer.
William married Helen Bostwick (Walker) Sinclaire (14 Nov 1862 - 21 Aug 1946) on 18 Jan 1899.