June 1536, Petre appeared in Convocation and made a novel claim to preside over its deliberations, on the ground that the King was supreme head of the church, Cromwell was the King's vicegerent, and he was Cromwell's deputy. After some discussion, his claim was allowed. In 1537 was employed to examine Robert Aske and other prisoners taken in the Lincolnshire and Yorkshire rebellions. Soon he was actively visiting and aiding in the suppression of the smaller monasteries; he was one of the most zealous of the visitors. Among the twenty monasteries he visited and procured the surrender of in 1538 were, perhaps, St. Leonard's, Thoby and Blackmore. In the first three months of 1539, thirteen more fell before him; he being instrumental in the almost total extirpation of the Gilbertines, the only religious order of English origin. A few years later, he was Visitor of the greater monasteries in Kent and the South of England. Secretary of State to Henry VIII In 1539, Petre was one of those appointed to prepare a bill for the enactment of 'The Six Articles', and in the following year was on the commission that declared the nullity of Henry's marriage with Anne of Cleves. Early in 1543, he was knighted, appointed Secretary of State in Wriothesley's place and placed on various commissions to inquire into causes of supposed heresy. In 1544, Henry made William a Privy Councillor, and on 9 July 1544, one of his two Principal Secretaries selected to assist Queen Catherine in carrying on the Regency in the small 'Regency Council' during Henry's absence, and to raise supplies for the King's expedition to Boulogne. In 1545, he was sent abroad as ambassador to the great Emperor Charles V, but at the end of the year was summoned back to the Privy Council. Though much occupied with public business, this astute politician found time and many opportunities to get and lay by great gain for himself. As one of the Visitors of the monasteries, he knew better than many people what properties were worth acquiring. He paid the King some £849 12s 6d (approximately £330,000 today) for the property known as "Ginge Abbes" at Ingatestone, even though the King was trying to raise money and this was a "fair" price it is probably true Sir William got a bargain. Ingatestone, which had previously belonged to Barking Abbey, must have been selected as being