Croisdale-Appleby joined the
Foreign and Commonwealth Office (FCO) in 1996 and served as
Principal Private Secretary to the Foreign Secretary 2010–2012. He also worked in the
Afghanistan Emergency Unit and as Desk Officer for
Nigeria. He then served as Ambassador to Colombia from November 2012 to 2015 before taking up an appointment as Europe Director in the Foreign Office. He was promoted to Director-General, EU Exit at the Foreign Office in 2017. He was Deputy Sherpa and Deputy Chief Negotiator in Taskforce Europe (10 Downing Street) throughout 2020. On 21 January 2021, he was appointed to succeed
Sir Tim Barrow as Head of the UK Mission to the European Union. It was announced that Croisdale-Appleby would be leaving his role as EU Ambassador in 2026, to be replaced by
Caroline Wilson. In September 2025, it was reported that Croisdale-Appleby was one of the front-runners from within the Diplomatic Service to succeed
Peter Mandelson as British Ambassador to the United States, or as
Permanent Representative of the United Kingdom to the United Nations, succeeding
Barbara Woodward. ==Personal life==