She was laid down, with the yard number 1285, as a Rothesay-class frigate to be called Fowey on 19 October 1959, but in 1960 it was decided to complete the ship as one of the new Leander class, with the new name Ajax.

Two oil-fired boilers fed steam at 550 pounds per square inch (3,800 kPa) and 850 °F (454 °C) to a pair of double reduction geared steam turbines that in turn drove two propeller shafts, with the machinery rated at 30,000 shaft horsepower (22,000 kW), giving a speed of 28 knots (52 km/h; 32 mph).

[10] Activities included taking part in the Beira Patrol, covering the withdrawal of British forces from operations off Aden, and acting as guardship for Hong Kong.

[11] In 1970, Ajax became the Gibraltar guard ship,[10] a required deployment at that time due to the tense fears of invasion by General Franco.

In September 1970, Ajax began a modernisation at Devonport Dockyard that lasted until 1973,[10] having her 4.5-inch gun turret replaced by an Ikara anti-submarine missile system.

A pair of quadruple GWS22 SeaCat launchers were fitted aft while the two Bofors guns were retained but moved forward to abreast the ship's mainmast.

She participated in further deployments that culminated in the highlight of her final year in 1985 when she escorted HMY Britannia, which took a number of the Royal Family on a tour of Italy.