Legion was ordered on 31 March 1938 from the yards of Hawthorn Leslie and Company, Newcastle upon Tyne under the 1937 Naval Estimates.

She was commissioned on 19 December 1940 at a total cost of £445,684, which excluded items such as weapons and communications equipment supplied by the Admiralty.

On completion of repairs, Legion was assigned to the Western Approaches Command at Greenock as part of the 11th Escort Group.

She was deployed on convoy defence duties, and also successfully trialled a modified Radar Type 286M using a rotating instead of fixed aerial array.

On 13 April she rescued survivors from the armed merchant cruiser Rajputana which had been torpedoed in the North Western Approaches by the German submarine U-108.

In September she and her flotilla returned to Gibraltar and resumed escorting capital ships supplying aircraft to Malta.

On her return to Gibraltar after Halberd, Legion and Gurkha attacked and sank the Italian submarine Adua with depth charges.

She made an unsuccessful attack on U-205 on 23 October and then rescued survivors from Cossack which had been torpedoed by U-563 west of Cape Spartel.

On 13 December she was part of the fleet when it intercepted the Italian cruisers Alberto di Giussano and Alberico da Barbiano.

After this success the ship was deployed with Force K, to carry out attacks on Axis convoys on passage in the central Mediterranean in support of military operations.

She then returned to Alexandria on 19 December with Force C. When the anti-submarine boom was raised to allow the ships to enter the harbour, the Italian submarine Sciré and three human torpedoes were able to penetrate into the secure anchorage.

They laid explosive charges, severely damaging the battleships Queen Elizabeth and Valiant, and the tanker Savona.

On 28 December Legion and Kipling sank U-75 off Mersa Matruh after a two and half hour hunt following the sinking of the merchant vessel Volo.