John Baylis was born in Barry in 1946 and attended Barry Boys Grammar School. He was a keen sportsman in his youth, joining Cardiff City at the age of 16, and subsequently playing for Barry Town and the University of Wales. He was also keen on cricket and athletics. He went to the University of Wales, Swansea, graduating in 1967 from the Department of Political Theory and Government. Following this he obtained an MSc (Econ) degree in Strategic Studies from the University of Wales, Aberystwyth. He spent a year teaching at Liverpool University before returning to the International Politics Department at Aberystwyth University as a Lecturer. He obtained his PhD, a DLitt., and a Personal Chair while teaching at Aberystwyth. He also spent two periods as Dean of Social Sciences. In 2000 he was appointed to the Chair in his old Department in Swansea and in 2003 he became a Pro Vice Chancellor at the University, retiring in 2008.
His research was in the related fields of Strategic Studies, International Relations and Nuclear History. With Aberystwyth colleagues, John Garnett, Ken Booth and Phil Williams, he published the first major textbook on Strategic Studies, Contemporary Strategy: Theory and Policies in 1975. While at Aberystwyth and Swansea he went on to publish more than 20 books and over a hundred chapters and articles. His books include Anglo-American Defence Relations 1939–1984 (Macmillan, 1984); Anglo-American Relations since 1939: The Enduring Alliance (Manchester University Press, 1997); Alternative Nuclear Futures: The Role of Nuclear Weapons in the Post-Cold War World, edited with Robert O’Neill (Oxford University Press, 2000); The Makers of Nuclear Strategy, edited with John Garnett (Pinter, 1991); An Introduction to Global Politics, with Steven Lamy, Steve Smith, and Patricia Owens (4th ed., Oxford University Press, 2016); The British Nuclear Experience: The Role of Beliefs, Culture and Identity, with Kristan Stoddart, (Oxford University Press, 2015; Wales and the Bomb: The Role of Scientists and Engineers in the British Nuclear Programme, (University of Wales Press, 2019), used as the basis for three programmes by Elin Rhys for BBC Wales in 2024); Joining the Non-Proliferation Treaty, edited with Professor Yoko Iwama, (Routledge, 2019); and Sharing Nuclear Secrets: Trust, Mistrust, and Ambiguity in Anglo-American Nuclear Relations since 1939, with Anthony Eames, (Oxford University Press, 2023). He is also one of the Editors of two major textbooks. One with Professor Sir Steve Smith and Professor Patricia Owens, The Globalization of World Politics: An Introduction to International Politics. The 10th edition will be published by Oxford University Press in 2025. The other text, Strategy in the Contemporary World, with Professor James J Wirtz and Professor Jeannie L Johnson, the 8th edition of which will be published by Oxford University Press in 2026.
Apart from continuing his research in retirement he returned to his interest in sport by becoming a member of the Board of Sport Wales for six years. He is a Fellow of the Learned Society of Wales, the Royal Historical Society and the Academy of Social Sciences.