In 1670 he was appointed Groom of the Stole to the Duke of York, and on 24 February 1673 ambassador extraordinary to arrange the terms of his proposed marriage with the Archduchess Claudia Felicitas of Innsbruck. He had hardly crossed the Channel, however, when the news of the Emperor Leopold I's determination to marry the archduchess himself put an end to the project. He was then commissioned to ascertain the respective personal and other attractions of the Princess Mary of Modena, and several other ladies between whom the duke's choice lay, and Mary having been fixed upon, proceeded to Modena in the following August as ambassador extraordinary to arrange the match. After some demur on the grounds of religion, Pope Clement X refusing a dispensation for the marriage of the princess with a prince who was not a declared Catholic, the scruples of the family were overcome, Peterborough being proxy for the duke (30 September 1673). Peterborough then escorted the princess to England.