Harris began his career at the Royal Liverpool University Hospital in 1991 as a Pre-Registration House Officer (junior doctor). A year later, he moved to the John Radcliffe Hospital, Oxford, specialising in acute medicine and surgery. In 1994 Harris moved to Oxfordshire Health Authority, becoming an honorary specialist registrar in public health and working on issues to do with NHS staffing and training. Harris held the position of local British Medical Association representative and negotiator from 1992 to 1994, following which he was elected to the BMA's National Council.