On the question of prerogative submitted to the judges in January 1718, whether the custody of the royal grandchildren was vested in the Prince of Wales or the king, Pratt concurred with the majority of his colleagues in favour of the Crown. He was one of the commissioners of the great seal in the interval (18 April–22 May 1718) between the resignation of Lord-chancellor Cowper and the seal's transference to Lord-keeper Parker (later the Earl of Macclesfield). He succeeded the latter, 15 May, as lord chief justice of the court of king's bench, being sworn of the Privy Council on 9 October.