[1] Following in the footsteps of his father, he was admitted to the Middle Temple on 14 November 1739, but was not Called to the Bar.
[1][3] On 3 March 1759 he was commissioned as lieutenant-colonel of the Surrey Militia which his kinsman Richard Onslow, 3rd Baron Onslow, had raised and briefly commanded as Lord Lieutenant of Surrey.
The militia was disembodied in December 1762 at the end of the Seven Years' War and the two Surrey battalions amalgamated on 26 February 1763, when Onslow was made colonel of the combined regiment.
On 20 May 1776, he was created Baron Cranley, of Imber Court in the County of Surrey.
They had four sons and one daughter:[1][3] In 1778, Lord Onslow sold Imber Court in Thames Ditton, and a large estate including farms and gardens, and the Imber Court Copper Mills.