In 1734, Onslow was returned as Member of Parliament for Guildford, holding that position until 1740.
On 16 May 1741, he married Mary Elwill (d. 19 April 1812), daughter of Sir Edmund Elwill, 3rd Baronet at her mother's residence in Clifford Street in Mayfair;[2] the marriage was childless.
from the University of Cambridge in 1749 and was made a Knight Companion of the Order of the Bath in 1752.
Upon his death in 1776, he was succeeded by special remainder in the barony by his second cousin, George Onslow.
[1] This biography of a baron or baroness in the Peerage of Great Britain (1707–1800) is a stub.