Cornwell was born in the United Kingdom to Ann Martin and David Cornwell, who had met as teenagers and had three sons. The couple later divorced. David Cornwell remarried and had a fourth son.[2]
Growing up, Cornwell traveled around Europe before settling in Crete, a Greek island, where his father pursued a career as a writer. He attended a rural private school before going on to Westminster School in London.[1]
Cornwell attended the University of Oxford, where he studied modern languages.
He later worked on the Thai-Laos-Cambodia border for the International Rescue Committee. He helped build Khao-i-dang, a temporary refugee camp.
He also studied management at the University of Yale in the United States.[1]