Mansfield grew up in Camberwell. During World War II he was evacuated from London, initially to Sevenoaks and then twice to Torquay, Devon, where he was able to stay with the same family on both occasions. On returning to London after the war he was told by a school master to take the 11+ exam. Having never heard of the exam before, and having no time to prepare, Mansfield failed to gain a place at the local Grammar school. His mark was, however, high enough for him to go to a Central School in Peckham. At the age of 15 he was told by a careers teacher that science wasn't for him. He left school shortly afterwards to work as a printer's assistant.