On 23 March 2015, it was announced that Arnold would be the next British Ambassador to Mongolia in succession to Christopher Stuart. She took up the appointment in May 2015. In March 2018 she was replaced by Philip Malone. From 2018, she served as head of the Illegal Wildlife Trade Unit at the Foreign and Commonwealth Office, and then led the United Kingdom's successful bid to chair the 2021 United Nations Climate Change Conference, COP26.