In June 1942, much of the Home Fleet was detached to the Mediterranean to take part in Operation Harpoon, an attempt to run one supply convoy to Malta from the west, while a second convoy headed to Malta from Egypt (Operation Vigorous). "Onslow" was part of Force W, the covering force which also included the battleship , the aircraft carriers and and three cruisers. Force W joined the Harpoon convoy shortly after it entered the Mediterranean on 12 June, and remained with it until turning back, as planned, as the convoy reached the Strait of Sicily on the evening of 14 June. After her return to British waters, "Onslow" formed part of the distant escort for Arctic convoy PQ 17 and the return convoy QP 13. PQ 17 was ordered to be scattered and its close escort withdrawn based on mistaken intelligence that the "Tirpitz" was about to attack the convoy, which resulted in extremely heavy losses to the unprotected merchant ships, with 24 of the 25 merchant ships being sunk by German submarines and aircraft. 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.