Onslow was born in 1904 at Garmston (near Ironbridge), Shropshire, second child and eldest son of George Arthur Onslow, farmer, and his wife Charlotte Riou, daughter of clergyman the Reverend Riou George Benson [REF].
In 1932 he married Kathleen Meriel Taylor, daughter of Edmund Coston Taylor, cotton manufacturer, of Bank House, Longnor, Shropshire; they had two sons [REF].