He studied philosophy at Street David"s University College, Lampeter, and also read and completed an Master of Arts in psychology at Selwyn College, Cambridge.
He became a psychologist for the Ministry of Defence, lecturing on the selection and training of air crews for the Royal Air Force at Brampton. In addition to writing he also worked as a teacher and later for the Scientific Civil Service working on aviation problems. He is known for writing four historical novels set in Roman and early medieval Britain and Europe.
Neil Gaiman is an admirer of James, especially his novel Votan, which provided one model for American Gods, calling it “I think probably the best book ever done about the Norse”.
The British writer Byron Rogers said that whereas "other historical novelists cheat" by putting people with modern opinions and sensitivities into the past, John James "had a man from the past as his hero". James "knew how such a man would have dressed and what he would have eaten, and, what is far more important, he knew what went on in his head".
James is buried in the graveyard at Strata Florida Abbey in Wales.