King Charles V’s (not Philip II's) ambassador to the Court of Rome 1547-1554.
Diego Hurtado de Mendoza (1503–1575) was a member of the Spanish nobility, remembered today as a novelist, poet, diplomat and historian. After studies at the University of Salamanca, Mendoza attended universities in Bologna, Padua and Rome. In 1537 he was sent on a failed mission to England to arrange marriages between Henry VIII and the Duchess of Milan, and between Prince Louis of Portugal and Mary Tudor. In 1539 was made ambassador at Venice, where he took advantage of the ancient libraries and archives to add to his burgeoning private library. Thereafter he was the military governor of Siena, represented Spain at the Council of Trent, and was ambassador to Rome from 1547 to 1554. It was only in 1556, after his ambassadorship, that Philip II became king. A dispute with one of Philip's courtiers resulted in his banishment from court in June 1568. The rest of his life was spent in Granada, devoted to the study of Arabic history and poetry.
By reason of this great devotion, and the great virtues of my Cid, and the miracles which were wrought by him, King Philip the Second gave order to his ambassador Don Diego Hurtado de Mendoza, to deal with the Court of Rome concerning the canonization of this venerable knight Rodrigo Diaz.
Now Don Diego was a person of great learning, and moreover, one of the descendants of the Cid; and being greatly desirous that this thing should be effected, he sent to the Monastery of St. Pedro de Cardena, and had papers and depositions sent from thence, and made a memorial of the virtues and miracles of the Campeador, showing cause why this blessed knight should be canonized.