After leaving school, Greenhill went to sea. He distinguished himself in the merchant service in the West Indies and was rewarded by the Admiralty. He was appointed Governor of the Gold Coast by the Royal African Company. In 1685 he was elected an elder brother of Trinity House, in 1689 a commissioner of the Transport Office, and in 1691 one of the principal commissioners of the navy, based at Plymouth and then at Portsmouth. The building of Plymouth Dockyard was completed under his direction. He received a mourning ring under Samuel Pepys's will.[1]