Mansfield left school aged 15 and worked as a printer before joining the army. He only entered academic life after studying part-time to complete his school exams. He then earned a bachelor degree and doctorate at the University of London. After a spell in the United States he started work at the University of Nottingham, eventually becoming Emeritus Professor of Physics there.

Mansfield played a key role in the development of MRI, a scanning technique that uses a strong magnetic field to reveal internal organs and other tissues in extraordinary detail. Mansfield showed how the signals emitted by the body in response to the magnetic field could be mathematically analysed to produce a clear image. He also demonstrated how to achieve extremely fast imaging.

MRI is important for medical diagnosis, treatment and follow-up. In many cases it has eliminated the need for surgery. Worldwide, more than 60 million investigations are performed each year using MRI.

Mansfield was elected Fellow of the Royal Society, London, in 1987 and received a knighthood in 1993.