Born in Batley, West Yorkshire, Cox graduated from the University of Cambridge in 1995, before working as a political assistant, then joining the international humanitarian charity Oxfam, where she rose to become its head of policy. She was selected to contest the Batley and Spen parliamentary seat after the previous incumbent, Labour's Mike Wood, decided not to stand in 2015. Having retained the seat for Labour she became a campaigner on issues relating to the Syrian Civil War, and founded the All-Party Parliamentary Group Friends of Syria, of which she became chair.