John obtained his BSc in Biological Sciences and his PhD from Essex University (UK). After a postdoctoral appointment at the University of Sussex, he was appointed to a lectureship at The Royal Post-Graduate Medical School (London). In 1987, he moved to the Department of Medicine at Cambridge University (UK) and in 1997 was awarded a Readership in Molecular Virology in the Department and, subsequently, a personal chair in 2005.
From 1980-1983, he was a Postdoctoral Research Fellow at the University of Sussex, working with Professor James H Sang on development of DNA mediated gene transfer in Drosophila in tissue culture cells (SERC)
From 1983-1986 he was a Postdoctoral Research Fellow at the University of Sussex, working with Professor J H Sang on cloning of the shibire gene in Drosophila and analysis of DNA sequences important for control of gene expression by gene transfer in Drosophila cells (SERC). From 1986-1987 he was a non-clinical Lecturer in the Departments of Medicine and Virology, Royal Postgraduate Medical School, Hammersmith Hospital, London and then a non-clinical Lecturer in the Department of Medicine, University of Cambridge Clinical School, Cambridge from 1987-1998.
From 1998-2005 he was a reader in Molecular Virology, Department of Medicine, University of Cambridge Clinical School, Cambridge. Since 2005, he has been Professor of Molecular Virology and, since 2017, Deputy Head of the Department of Medicine.