The newly purchased vessel was sailed to Deptford Dockyard in late 1682, for fitting out as a Royal Navy ship of the line. She remained at Deptford throughout 1683, finally being commissioned into active service on 20 May 1684 under the command of Captain Henry Killigrew. The War of the Reunions had broken out in Europe, but Britain was at least temporarily at peace and "Mordaunt" was sent to cruise along the coastline of West Africa as protection for Britain's regional British merchant and slave-trading interests. She returned to England in 1685, where she remained for the next two years. Killigrew left the vessel in 1687, with command transferring to Captain John Ashby.