The son of Thomas, First Baron Pelham of Laughton and Lady Grace Holles. He was the older brother of Henry Pelham. Newcastle was the most prominent of the Old Corps Whig who had stayed in office throughout the reigns of the first two King Georges. This would lead to difficulties with the far less political King George III. Thomas would find it difficult to hold together the loose coaltion of Whigs without the unflagging loyalty of the monarch.