He was born in Southampton, Hampshire, England on 30 August 1875. He was the son of William Henry Slight (b. 1836), a house decorator, and Mary Bryant (b. 1837). His father hailed from Southampton and his mother from Ashton, Kent and they had married in Sussex in 1864. William had six known siblings: Clara Ann (b. 1866), Jessy Kate (b. 1868), Alice Mary (b. 1870), Henry James Bryant (b. 1871), Mary Jane (b. 1873) and Harry John (b. 1878). William's wife Kate died at the beginning of 1912.