In February 1720, Onslow had been returned as
Whig Member of Parliament for
Guildford at a by-election. He represented that borough until Parliament was dissolved in 1727. During this period, he was known to have declared against a proposal to levy
Roman Catholics in 1722, and opposed the motion to reverse
Bolingbroke's attainder in 1725. In 1726, he was one of the Commons managers for the trial of
Macclesfield for corruption. 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. He would be unanimously re-elected Speaker in 1735, 1741, 1747 and 1754, setting a record for length of service in that office: 33 years. On 25 July 1728, he was sworn of the
Privy Council, and was also made a
bencher of the
Middle Temple that year. The following year, on 13 May 1729, he was made Chancellor and Keeper of the Great Seal to
Queen Caroline, who was godmother to his son George in 1731. In October 1731 the
Cotton Library caught fire. Onslow's intervention on the night, throwing books out of the windows into the courtyard below, and subsequent oversight of their repair and preservation, saved many valuable books and manuscripts for the nation. Onslow's speakership was distinguished by his great integrity in a corrupt and jobbing age. His great achievement as Speaker was to assert the independence, authority, and impartiality of that post. While he continued to participate in ordinary political activity, speaking and voting in committee, he did not hesitate to oppose Government policy when necessary. Onslow saw his role to be the protection and defence of Parliament in the tradition established by the
Glorious Revolution. He insisted on rigidly observing parliamentary forms and procedure, which he viewed as a protection to independent MPs. But his devotion to precedent led him to condemn the reporting of parliamentary debates as breach of privilege. On 20 April 1734, Onslow received the valuable office of
Treasurer of the Navy. In 1742, after casting his vote on a highly political issue, he resigned the post to confute claims of political influence. He continued to receive emoluments from the office of Speaker, such as those for private bills. He also became
Recorder of
Guildford and high steward of
Kingston upon Thames in 1737. During the 1730s, he was involved in the effort that led to the charter of the
Foundling Hospital and was one of its founding governors. In 1753 he was instrumental in persuading Parliament to fund the purchase of
Hans Sloane's collection and the
Harleian Library, which together with the
Cotton Library established the
British Museum. Due to failing health, he retired from Parliament in 1761, doing so with the unanimous thanks of the House of Commons. He received an annuity of £3000 from the King for his life and that of his son, the first occasion upon which a retiring Speaker was pensioned. ==Legacy and honours==