Cafodd Shan ei phenodi’n Ddirprwy Gynrychiolydd Parhaol y DU i’r UE ym mis Mehefin 2012. Yn y rôl hon, bu’n gyfrifol am gyflawni blaenoriaethau’r DU ar draws ystod o feysydd polisi’‘r UE sy’n cael effaith uniongyrchol ar fuddiannau economaidd Prydain mewn partneriaeth agos â’r Gweinyddiaethau Datganoledig. Cyn hynny, treuliodd bedair blynedd fel Llysgennad Prydain i’r Ariannin a Paraguay (2008-2012). Yn 2006, trosglwyddodd Shan i’r Gwasanaeth Llysgenhadol fel Cyfarwyddwr yr UE yn y Swyddfa Dramor, a hi oedd yn gyfrifol am drafod Cytundeb Lisbon a rheoli proses gadarnhau seneddol y DU. Ymunodd Shan â’r Gwasanaeth Sifil ym 1977, a chanolbwyntiodd ei gyrfa gynnar ar bolisi cyflogaeth a materion cymdeithasol.
Shan was appointed as UK Deputy Permanent Representative to the EU in June 2012. In this role she has been responsible for delivering UK priorities across a wide range of EU policy areas which directly affect British economic interests in close partnership with the Devolved Administrations.  Prior to that, she spent four years as British Ambassador to Argentina and Paraguay (2008-2012). In 2006 Shan transferred to the Diplomatic Service as EU Director in the Foreign Office, and was responsible for negotiations on the Lisbon Treaty and management of the UK parliamentary process of ratification. Shan joined the Civil Service in 1977 and her early career focussed on employment and social affairs policy.