Purefoy was born 3 June 1964 in Taunton, Somerset, the eldest son of Anthony Chetwynd Purefoy and Shirley, née Taylor.
Purefoy boarded at Sherborne School, which he left with only one O-level. He later went to night school and received 11 more O-levels, before taking his A-levels at Brooklands College in Weybridge. He worked as a porter at Yeovil District Hospital before studying acting at the Central School of Speech and Drama.
Purefoy said that as a “young radical actor”, he had different politics to his father, who was a Tory and who worked for the Conservative Party; they had had “long interesting discussions” about politics.