After commissioning and workup, "Leeds Castle", assigned the Pennant number P258, started fishery protection patrols around the Shetlands and in the English Channel in March 1982. On 2 April 1982, the Falklands War began when Argentine forces invaded the Falkland Islands. "Leeds Castle" was diverted from her normal North Sea patrol duties on 22 April, being fitted additional communications equipment and a desalination plant at HMNB Portsmouth before setting out for the Falklands on 29 April. She arrived in the Total Exclusion Zone on 21 May where she was employed as a despatch vessel, duties including acting as a refuelling platform for helicopters transferring stores between the transports "Queen Elizabeth 2", and near South Georgia on 27 May. She was then ordered to Ascension Island to serve as Guardship, arriving there on 6 June 1982, before setting out again for the Falklands on 29 June, arriving at Port Stanley on 12 July. She set out on her return journey to the Britain on 24 July, arriving back at her home port of Rosyth on 20 August 1982.