William Arthur Bampfylde Onslow, was born on 11 June 1913 and was a British peer and soldier.

William was the eldest son of the 5th Earl, and was educated at Winchester and Sandhurst. He was a Lieutenant in the Life Guards during World War II. He inherited his father's titles in 1945 and was a county councillor for the County of London from 1940-46 and for Surrey from 1949-42.

On 4 August 1936, he married Pamela Louisa Eleanor Dillon, daughter of Eric Dillon, 19th Viscount Dillon and they had two children, Michael William Coplestone Dillon, and Teresa Lorraine. William Onslow was knighted in 1960; he divorced his wife in 1962 and married Nina Edith Jo Sturdee on 30 June that year.

He died on 3 June 1971 and was succeeded by his only son.