The Leander class was influenced by the York-class heavy cruiser and was an attempt to better provide for the role of commerce protection.

The 7,000-7,200 ton Leanders were armed with eight BL 6 inch Mk XXIII naval guns in twin turrets, two forward and two aft.

Their close-range anti-aircraft weaponry consisted of twelve 0.5-inch (13 mm) Vickers machine guns in three quadruple mounts.

They also shipped a bank of four 21-inch (530 mm) torpedo tubes on each beam and provision was made in the design for the carriage of two catapult-launched Fairey Seafox aircraft.

[1] The first five vessels did not contain dispersed machinery; the boiler rooms were arranged together and exhausted into a single funnel, a unique feature amongst British cruisers.

All demands on the machinery were met more than adequately, all material standing up to the strain in such a manner that nothing but confidence was felt during the action.

When all the machinery of the Achilles had worked up to full power, readings gave a total of almost exactly 82,000 horse-power, with the four propellers turning at an average of 283 revolutions a minute.’ This tribute to the soundness of design and the excellence of British shipyard workmanship is underlined by the statement of Captain Woodhouse of the Ajax that steam had been shut off the main engines of his ship for only five days since 26 August 1939.

At the Battle of Kolombangara, Leander was heavily damaged by a Long Lance torpedo, causing many casualties, and sending the ship to repairs for two years.

[6] During design, it was planned to modify the forward-most and aft-most 6-inch turrets to be fitted with three guns instead of two, but the plan was cancelled when it was determined that the required alterations would cause several negative side effects, including reducing the ship's top speed and causing problems with effective fire control.

Laid down as HMS Phaeton, the ship was acquired by the RAN, launched as HMAS Sydney and was commissioned in 1935.

Douglas Reeman's novel A Ship Must Die is set aboard the titular fictional Leander-class cruiser Andromeda pitted against a German surface raider operating in the Indian Ocean.