J.A. North taught Greek and Roman History in the Department of History at UCL, from 1963, and became Professor and Head of Department in 1992. He retired as Emeritus Professor in 2003. From 2014 to 2016, North was the Director of the Institute of Classical Studies. From an early stage in his career, North's research centred on the Religious History of Rome from the early Republic onwards; later, on the relations between pagan religious society under the Empire and the emergence of the Christian religion; more recently still, on the religious activities of Roman slaves and freed slaves.

The Religious History of the Roman Empire: The Republican Centuries is the second Oxford Readings in Classical Studies volume on the religious history of the Roman Empire, accompanying the volume on paganism, Judaism, and Christianity. This volume presents fourteen chapters dealing with aspects of the religious life of Republican Rome between c. 500 BCE and the fall of the Republican constitution in c. 30 BCE. The topics covered include Iron Age
rituals (Christopher Smith); Roman Priesthood (John Scheid; Mary Beard); religion and war (Jörg Rüpke); religious behaviour in the context of polytheism (Andreas Bendlin); religious ritual in early and middle Republic (John North); Italian warfare practices (Olivier de Cazanove); the role of women (Rebecca Flemming);
sacrificial ritual in Roman poetry (Denis Feeney); the centuriation-ritual (Daniel Gargola); Roman divination (Mary Beard); Augustan Peace and the stars (Alfred Schmid); the great cult-places of Italy (John Scheid); the grove of Pesaro (Filippo Coarelli). Originally published between 1981 and 2011, these chapters provide a vivid picture of key issues under discussion in this period, providing a missing link in the historiography of Roman republican religion. A central question concerns the
balance to be found between ritual and belief, both problematic concepts in interpreting this religious tradition. While there can be no question that the performance of rituals was a regular traditional activity to which Romans attached great significance, particularly those who were in a responsible
position as priests or senators, the later years of the Republic increasingly saw religious issues taken as matters for debate, and books on religious themes, unknown before the age of Cicero and Varro, began to appear.