The second son of Sir Christopher Heydon, in 1613 he was keeper of the stores in Sandown Castle, Deal, Kent.
In 1627 he was appointed lieutenant of the ordnance in the place of his brother Sir William, who was killed in the expedition to the Isle of Rhé.
[1] Between 1627 and 1643 Heydon was actively occupied in furnishing men, provisions, arms, guns, and ammunition for the service of the king.
When Charles I raised his standard at Nottingham on 22 August 1642, Heydon was in charge of cannon and ammunition at York.
He acted as Lieutenant-General of the Ordnance with Charles's forces, and joined his Privy Council.