In 1727, he was returned both for Guildford and Surrey, with the highest majority ever recorded, and elected to serve for Surrey; his younger brother Richard Onslow replaced him at Guildford in a by-election.
On 23 January 1728, Onslow was unanimously elected Speaker of the House of Commons, a post which had been held by his uncle Sir Richard Onslow, Bt and his ancestor Richard Onslow.