Warren Brown is interested in the social and political history of medieval Europe. His research follows two tracks. The first concerns the use of written documents in Europe before the turn of the first millennium. Written documents in this period have traditionally been considered the domain of the Christian clergy; Brown is interested in how lay people used documents beyond their interactions with churches or monasteries. This ongoing interest in studying the sources of history themselves led him to join a group of scholars in North America, the United Kingdom, and Europe who wanted to investigate evidence of people using documents outside the context of the church. As this project developed, he independently contributed three articles that reach positive conclusions about lay document use in Europe north of the Alps in the eighth and ninth centuries. Together, the group published their work in the book Documentary Culture and the Laity in the Early Middle Ages (Cambridge, 2013). Brown used the sources he worked with in this project to explore the social history of the laity in the early Middle Ages, and, in 2022, he completed his latest book, entitled Beyond the Monastery Walls: Lay Men and Women in Early Medieval Legal Formularies (Cambridge, 2022).

Brown's second research track concerns violence in medieval Europe in general. His book Violence in Medieval Europe (Longman, 2011) examines the societal, cultural, and legal norms that governed personal violence between the years 600 and 1500. In it, he describes the shifting of those norms, which people used to legitimize or delegitimize violent acts. Drawing on current literature on social norms in anthropology and political science, Brown tracks in particular how norms of violence competed with one another and achieved dominance or disappeared—until the norms associated with a central authority's monopoly of violence began to prevail. He is currently exploring how terror was used in the Middle Ages to achieve specific goals and how such use may inform the understanding of terrorism today. Articles laying the groundwork for this effort have recently appeared in the Oxford Handbook of Terrorism (Oxford, 2019) and the Cambridge History of Terrorism (Cambridge, 2021).

In the winter and spring of 2019, Brown was a member of the School of Historical Studies at the Institute for Advanced Study in Princeton, NJ. In 2013, he received a Donald Bullough Fellowship in Mediaeval History at the Institute of Mediaeval Studies at the University of St. Andrews. He also received the Associated Students of Caltech (ASCIT) Teaching Award for excellence in teaching in 2003 and again in 2006.