Following his PhD, Mansfield was invited to postdoctoral research with Charlie Slichter at the University of Illinois at Urbana–Champaign, where he carried out an NMR study of doped metals.
In 1964, Mansfield returned to England to take up a place as a lecturer at Nottingham University where he could continue his studies in multiple-pulse NMR. He was successively appointed Senior Lecturer in 1968 and Reader in 1970. During this period his team developed the MRI equipment with the help of grants from the Medical Research Council. It was not until the 1970s with Paul Lauterburs and Mansfields developments that NMR could be used to produce images of the body. In 1979 Mansfield was appointed Professor of the Department of Physics until his retirement in 1994.
* 1962: Research Associate, Department of Physics, University of Illinois
* 1964: Lecturer, Department of Physics, University of Nottingham
* 1968: Senior Lecturer, Department of Physics, University of Nottingham
* 1970: Reader, Department of Physics, University of Nottingham
* 1972–73: Senior Visitor, Max Planck Institute for Medical Research, Heidelberg
* 1979: Professor, Department of Physics, University of Nottingham
Mansfield is credited with inventing slice selection for MRI - i.e. the method by which a localised axial slice of a subject can be selectively imaged, rather than the entire subject - and understanding how the radio signals from MRI can be mathematically analysed, making interpretation of the signals into a useful image a possibility. He is also credited with discovering how fast imaging could be possible by developing the MRI protocol called echo-planar imaging. Echo-planar imaging allows T2* weighted images to be collected many times faster than previously possible. It also has made functional magnetic resonance imaging (fMRI) feasible.
Whilst working at Nottingham University, Mansfield tested the first full body prototype, installed just before Christmas, 1978. Mansfield was so keen, that he volunteered to test it himself and produced the first scan of a live patient. The prototype machine is now an exhibit, in the Medical Section of the Science Museum.