Evan Harris was born in Sheffield, South Yorkshire, the son of South African Jewish parents (his father was a medical professor). He was brought up in Liverpool, where he had a state education at the Liverpool Blue Coat School. He won a scholarship to Harvard High School in California in 1984, and later won a scholarship to Wadham College, Oxford, where he was awarded a Bachelor of Arts degree in physiology and a diploma in medical sociology. He completed his education at the Oxford Medical School where he received a Bachelors of Medicine and Surgery and qualified as a doctor in 1991.

Evan 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 he moved to Oxfordshire Health Authority, becoming an honorary specialist registrar in public health and working on issues to do with NHS staffing and training. He held the position of local British Medical Association representative and negotiator from 1992–1994, following which he was elected to the BMA's National Council.

Evan is a humanist, and is a Vice President of both the British Humanist Association and Gay and Lesbian Humanist Association. He was also a vice-chair of the All Party Parliamentary Humanist Group, before his defeat in the 2010 general election. In addition, he is an Honorary Associate of the National Secular Society, and the Patron of the Oxford Secular Society.