Mullin joined Oxford Archaeology in 2008 and worked in the post-excavation department. Whilst there he was trained by Ann Woodward in the analysis of prehistoric pottery and, as well as writing numerous specialist reports, completed work on the prehistoric material from Cirencester before Corinium and on a Roman settlement site at Hassocks, West Sussex.
After completing a PhD at the University of Reading, supervised by Professor Richard Bradley, Mullin became a lecturer in Heritage and Archaeology at the University of Worcester, working there from 2008 to 2019. During this time, Mullin organised a session at the 2008 Theoretical Archaeology Group conference; he edited a collection of papers on the theme of borders and wrote the introduction (which drew on his doctoral research) and the conclusion.[3]
Mullin also worked at the Oxford University Department of Continuing Education, where he taught on the Undergraduate Certificate and Undergraduate Diploma in Archaeology for four years. He also acted as course coordinator for the Early Prehistory module of the Undergraduate Diploma and was Chair of the Exam Board for the Certificate courses.
In 2012, Mullin published his PhD thesis as A Landscape of Borders: the prehistory of the Anglo-Welsh borderland by Archaeopress. A review by Keith Ray for the Prehistoric Society assessed that the book "makes an important point-in-time contribution to research into, and awareness of, some critical classes (and groups) of prehistoric material. The work on hoards ... and on temper sourcing in particular, will provide a springboard for future important research".[4]
During 2020–21, he was employed as a Research Infrastructure Advisor by Historic England, where he helped to develop the Research Frameworks Network.
Mullin, David, ed. (2011). Places in Between: The Archaeology of Social, Cultural and Geographical Borders and Borderlands. Oxford: Oxbow Books. ISBN9781842179833.
Mullin, David (2012). A Landscape of Borders: the prehistory of the Anglo-Welsh borderland. British Archaeological Reports British Series. Vol. 572. Oxford: Archaeopress. ISBN978-1-4073-1057-2.
^Hooke, Della (2012). "Places in Between. The archaeology of social, cultural and geographical borders and borderlands (Oxbow Books, Oxford, 2011). Edited by David Mullin. 217 x 276 mm. 104 pp. 40 illustrations, 10 tables. ISBN 978 1 84217 983 3. Price £30.00". Landscape History. 33 (1): 104.