Swiftsure was commissioned into the Royal Navy in August 1755, under Captain Augustus Keppel. In 1756 her command was transferred to Captain Matthew Buckle, and she was assigned first to the fleet under Admiral Henry Osborn, and then to that of Edward Boscawen. In company with HMS Monmouth she engaged and captured the French ship of the line Foudroyant in 1758, and towed her to join the fleet of Admiral Osborn at Cartagena.[2]
In 1759 she was again with Admiral Boscawen at Lagos, and at Quiberon Bay later that year, and at the capture of Belle Île in 1761.[3]