From 1980-1995, he was Junior Professor at the University of Birmingham. From 1995 to 2000, he was Department Chair at the University of Birmingham. From 2000 to 2005, Haldon served as the Dean of Faculty at the University of Birmingham. He was a Senior Fellow at the Dumbarton Oaks Center for Byzantine Studies, Washington, DC from 2007-2013 and has been Professor of Byzantine History and Hellenic Studies at Princeton University since 2009. Haldon has also served as the Shelby Cullom Davis Professor of European History at Princeton since 2005 and the Director of Graduate Studies for the Princeton History Department Since 2009. In addition, he has served as the Director of the Mossavar-Rahmani Center for Iran and Persian Gulf Studies. He is the author and co-author of over 25 books including The empire that would not die: The paradox of eastern Roman survival, 640 – 740, A tale of two saints: the passions and miracles of Sts Theodore 'the recruit' and 'the general', A Critical Commentary on the Taktika of Leo VI and Byzantium in the Iconoclast Era: A History, with Leslie Brubaker. His current projects include a re-examination of the relationship between environment and social and political change in the Byzantine world; research on some aspects of institutional and administrative history of the seventh-eighth centuries; preparation of an English translation of and critical commentary on the tenth-century text De Thematibus; research into the origins and development of the so-called Second Iconoclasm; work on comparative state formation in pre-modern societies. He is also in the initial stages of a major project on the history of the Eurasian world ca 400-800.
Recruitment and Conscription in the Byzantine Army c. 550-950: A Study on the Origins of the Stratiotika, Österreichischen Akademie der Wissenschaften, 1979.
Byzantine Praetorians: An Administrative, Institutional, and Social Survey of the Opsikion and Tagmata, c. 580-900, R. Habelt, 1984.
State, Army, and Society in Byzantium: Approaches to Military, Social, and Administrative History, 6th-12th Centuries, Variorum, 1995.
Contributor to Virgil S. Crisafulli and John W. Nesbitt, editors, The Miracles of St. Artemio: A Collection of Miracle Stories by an Anonymous Author of Seventh-Century Byzantium, E. J. Brill, 1997.
Warfare, State, and Society in the Byzantine World, 565-1204, UCL Press, 1999.
Byzantium: A History, Tempus Publishing, 2000.
Byzantium in the Iconoclast Era : The Sources: An Annotated Survey, Ashgate, 2001.
The Byzantine Wars: Battles and Campaigns of the Byzantine Era, Tempus Publishing, 2001.
Editor, Archaeology and Urban Settlement in Late Roman and Byzantine Anatolia Euchaïta-Avkat-Beyözü and Its Environment, Cambridge University Press, 2018.
