Jekyll was returned as a Whig Member of Parliament for Eye at a by-election on 14 December 1697 and was returned again in the following year at the 1698 English general election. He was seen as part of the Whig Junto. He was extremely active in parliament and an excellent speaker. He played a role in drafting various bills and acts. Occasionally he voted against the party, mainly because he supported greater reform of the electoral system and the removal of bribery and corruption. He was returned unopposed for Eye in the two general elections of 1701 and in 1702 and 1705. Returned again at the 1708 British general election he was involved in the impeachment of Henry Sacheverell in 1709 and 1710. He was returned again for Eye at the 1710 British general election but at the 1713 British general election was returned instead for Lymington. At the 1715 British general election Jekyll was returned again for Lymington. He was asked to participate in the secret committee tasked with preparing the impeachment of the Earl of Oxford and the Duke of Ormonde, which he refused to support. He persuaded the government to open an investigation into the collapse of the South Sea Company in 1720. At the 1722 British general election he was returned as MP for Reigate where he was returned again in 1727 and 1734. He sponsored the Mortmain Act and the Gin Act 1736, and was noted for his opposition to intoxication, which annoyed the public so much that he was forced to have a guard at his house at all times. Under Robert Walpole he remained independent of the government in terms of how he voted, and was described by Alexander Pope as "an odd old Whig, who never change his principles or wig". Outside Parliament, Jekyll provided £600 to fund the colony at Jekyll Island, and as a result James Oglethorpe named the island in the Province of Georgia after him.
Master of the Rolls
On 13 July 1717, Jekyll was appointed Master of the Rolls, and the same year became a Privy Councillor. His time as Master "was distinguished by legal ability, integrity and despatch", and during this period he helped write The Judicial Authority of the Master of the Rolls. He was given the Great Seal on 7 January 1725, and held it until 1 June.
Death and legacy
On 19 August 1738 he died of "a mortification in the bowels", and was buried in the Rolls Chapel. He had no children. In his will he left £20,000 to help pay off the national debt, something Lord Mansfield described as "a very foolish bequest.. he might as well have attempted to stop the middle arch of Blackfriars Bridge with his full-bottomed wig". Jekyll Island is named in his honor.