During 1952–1958, Allen was a science officer and then senior science officer at the Atomic Energy Research Establishment (AERE, Harwell). From 1958–1964, he was a consultant and the Brazilian National Nuclear Energy Commission (CNEN) Laboratory and a professor at the University of Rome in Italy. During 1964–1965, he was an assistant director of research at the University of Cambridge at the invitation of the engineer W. R. Hawthorne. He spent the major portion of his career as a Fellow of University College, Oxford during 1965–1996 (31 years). He was a Reader (1990–1996) and then Emeritus Professor of Engineering Science from 1996 in the Department of Engineering Science at Oxford, when he became an Emeritus Fellow of University College. In 1991, he was editor of the book Gas Discharge Physics with the theoretical physicist Yuri Raizer. He has also been a Visiting Professor in Physics at Imperial College London.
