William Hillier Onslow was born at Bletsoe, Bedfordshire, the only son of George Augustus Onslow (1813–55), nephew of the Third Earl, and of Mary Harriet Ann, eldest daughter of Lieutenant-General William Loftus of Kilbride, County Wicklow.

He was educated at Eton, and Exeter College, Oxford, succeeding his great-uncle as Fourth Earl in 1870.

He was a British Conservative politician. Having succeeded as 4th Earl of Onslow, he held office in the governments of Lord Salisbury and Arthur Balfour. From 1889 until 1892 he served as Governor of New Zealand.

William Hillier Onslow died at his son's house in Hampstead on 23 October 1911.