"Implacable" was laid down by Fairfield Shipbuilding and Engineering Co. at their shipyard in Clydeside on 21 February 1939, as Yard Number 672. Her construction was temporarily suspended in 1940–41, in favour of higher-priority ships needed to fight in the Battle of the Atlantic, before she was launched on 10 December 1942 by Queen Elizabeth. Captain Lachlan Mackintosh was appointed to command the ship in November 1943. She was commissioned on 22 May 1944, and began sea trials which revealed a significant number of problems that required rectification, so the ship was not formally completed until 28 August. "Implacable" was assigned to the Home Fleet and was working up over the next several months while the Fairey Fireflies of 1771 Squadron flew aboard on 10 September. The squadron was followed by the Fairey Barracuda torpedo bombers of 828 and 841 Squadrons that made up No. 2 Naval Torpedo-Bomber Reconnaissance Wing later that month.