She sank the German submarine U-589 in September 1942 and in December that year took part in the Battle of the Barents Sea in 1942, while escorting Convoy JW 51B to Russia.

The convoy escorts held off attacks from the powerful Admiral Hipper, with Onslow being heavily damaged and her captain, Robert Sherbrooke, severely injured.

She also saw detached service in the Mediterranean, covering the Malta Convoy Operation Harpoon in June 1942, and protected invasion shipping in the English Channel from German attack before and after the Normandy landings in mid-1944.

The ship was designed to carry two quadruple 21-inch (533 mm) torpedo tubes, but early experience of the vulnerability of destroyers to air attack off Norway and during the evacuation from Dunkirk in 1940 resulted in the armament of the O-class being revised during construction, with the aft set of torpedo-tubes removed and replaced by a single 4 in (102 mm) QF Mark V anti-aircraft gun,[4][5] although the 4-inch gun was later removed and the second bank of torpedo tubes re-instated.

[6] Onslow was completed with a close-in anti-aircraft armament of one quadruple 2-pounder "pom-pom" mount together with four single Oerlikon 20 mm cannon, with two on the bridge wings and two further aft abreast the searchlight platform.

[10][11] The ship was ordered as part of the Second Emergency Flotilla as Packenham on 2 October 1939,[12] at a contract price of £416,770 (excluding government provided equipment such as armament),[13] and was laid down at John Brown & Company's Clydebank shipyard on 1 July 1940.

The force arrived at its destination on 27 December, and while Commandos landed on the islands, Onslow and Oribi attacked a coastal convoy, sinking driving aground four merchant ships (Reimar Edward Fritzen, Norma, Eismeer and Anita M Russ) and boarding the Vorpostenboot (an armed trawler) Föhn, capturing coding wheels and bigram tables for the Enigma cypher machine, before sinking Föhn.

More codebreaking material was captured later that day when Offa and Chiddingfold boarded and sunk the armed trawler Donner while sinking the cargo ship Anhalt.

[21][22] On 1 January 1942, Onslow rescued 23 survivors from the British merchant ship Cardita, torpedoed the previous day by the German submarine U-87.

[23] In early March 1942, Onslow sailed with the main body Home Fleet as part of the distant escort to the Arctic Convoys QP 8 and PQ 12.

[31][32] In August 1942, Onslow was employed in escorting units of the Home Fleet returning to British waters from Gibraltar after another Malta Convoy, Operation Pedestal.

[42] On 8 November 1942, the British and Americans landed in French North Africa in Operation Torch,[43] with Onslow being employed in escorting follow-up convoys following the initial assault.

Sherbrooke in Onslow led the other destroyers in dummy torpedo attacks against Hipper in order to force the cruiser to keep from closing, while laying a smoke-screen to protect the convoy.

Two guns were put out of action[f] and a serious fire started, while 17 members of Onslow's crew were killed, and 23 wounded, including Sherbrooke.

[23] On the night of 27–28 April 1944, nine German S-boats (motor torpedo boats) attacked a convoy of American landing craft on exercise in Lyme Bay, sinking two and damaging another.

[60][23] On 14 and 15 May 1944, Onslow formed part of the escort for the two aircraft carriers Emperor and Striker as they launched attacks against shipping in the ports of Rørvik and Stadlandet in German-occupied Norway, as part of a series of strikes by British aircraft carriers against Norway with the intention of distracting German attention from Northern France, as well as stopping German coastal shipping.

[63][66] On 12 August, Onslow, together with the cruiser Diadem and the destroyer Piorun sank the German auxiliary minesweeper Sperrbrecher 7 near La Rochelle.

They attacked a German convoy off Egersund, sinking the minesweeper M-273 and shelling the merchant ships Bahia Camarones and Charlotte, which were abandoned and sank.

[72] In February 1945, Onslow was part of the escort for Convoy JW 64, which came under heavy air and submarine attack, with 12 German bombers being lost in exchange for the corvette Denbigh Castle, which was torpedoed by U-992.

The return convoy RA 64 was attacked by German submarines on leaving the Kola Inlet on 17 February, with the sloop Lark and the freighter Thomas Scott torpedoed and sunk by U-968, and the corvette Bluebell by U-711.

Although the war in Europe had ended on 8 May, these last Arctic convoys were still provided with a substantial escort to guard against attacks from submarines that did not obey the German order to surrender.

[75] On 5–7 June 1945, Onslow escorted the cruiser Norfolk, carrying King Haakon VII of Norway back from exile to Oslo.

[50] In 1948, the newly established Pakistan Navy sought to acquire two 4.7-inch gunned destroyers from Britain, and purchased Onslow and Offa for a total price of £605,000 for the two ships.