Ian and his elder brother, the biographer Mark, were brought up in Streatham. The family lived at 163 The High. His parents, Leslie and Lilly both were from working-class families and had gone to grammar school but left in their mid teens. They were keen on sport – Bostridge's great-grandfather had played for Millwall and Spurs – but both boys were "terribly shortsighted, hopeless at sport and bookish" and won scholarships to Dulwich Prep School and then Westminster. "
Gained a PhD on 17th and 18th century witchcraft from Oxford