[1] Treby was returned unopposed as Whig Member of Parliament for the family's Rotten Borough of Plympton Erle at the 1708 general election, when he was in his early twenties.
He took an active part in debates of the House, and acted frequently as a teller on the Whig side.
He was again a frequent teller, and voted against the expulsion of Richard Steele,[2] Treby was returned unopposed as MP for Plympton Erle at the 1715 British general election and was a Commissioner for forfeited estates from June 1716 to 1719.
In 1724, Walpole wanted to give post of Secretary of War to Pelham, and got rid of Treby in 1724 making him Teller of the Exchequer.
At the accession of King George, Treby was deprived of his post in the Tellership, and at the 1727 British general election, he was returned as MP for Dartmouth.