Consecrated titular archbishop of Heraclea in 1885, he returned to Madrid as nuncio, but was shortly afterwards created cardinal and appointed to the papal secretaryship of state.
He voted for the exclusion of James, duke of York, from the throne, and made overtures to William, prince of Orange, and consequently in 168r he lost both his secretaryship and his seat on the privy council.
He held the secretaryship till November 1744.
In February 1746 he allowed himself to be entrapped by the intrigues of the Pelhams into accepting the secretaryship, but resigned in forty-eight hours.
In general European politics Baron Marschall had taken during his Foreign Secretaryship a strongly imperialist attitude.