Forester was born in Cairo and, after a family breakup at an early age, moved with his mother to London, where he was educated at Alleyn's School and Dulwich College, south London. He began to study Medicine at Guy's Hospital, London, but left without completing his degree. Forester had always worn glasses and been of slender physique. Trying to enlist in the army, he failed his physical and was told there was not a chance that he would be accepted, even though he was of good height and somewhat athletic. Around 1921, after leaving Guy's, he began writing seriously using his pen name.