Reid was born in Croydon, London to an English father, of Scottish ancestry and an English mother. Reid was educated at the independent Croham Hurst School in Croydon (from 1975–81) with the comedienne Sue Perkins, followed by the independent Croydon High School (1981–87) and St Paul's Girls' School (1987–89) in London before studying Politics, Philosophy and Law at the University of Bristol (1989–92). She was Editor of EpiGram the student newspaper when it was shortlisted for Best Student Newspaper in The Guardian/NUS student journalism awards 1991. Reid also has a Postgraduate Diploma in Broadcast Journalism from Cardiff School of Journalism.