Sullivan is emeritus distinguished professor of arts and sciences from East Carolina University. Among his other awards were a Distinguished Research Professor Award from the ECU Board of Trustees and a Distinguished Professor for Teaching Award from the UNC Board of Governors. In 1995, Sullivan was elected to the Welsh Academy for his contributions to the study of medieval Welsh Celtic myth and legend.

While at ECU, Sullivan published 10 books and over 50 refereed articles; he gave invited lectures at various institutions in the U.S. as well as in Wales, Ireland, England, Australia, and Hungary. His major publications include: Welsh Celtic Myth in Modern Fantasy, The Mabinogi: A Book of Essays, Fenian Diary: Denis B, Cashman on board the Hougoumont, 1867-1868, Cultural Worldview: Marginalizing the Fantastic in the Seventeenth Century, and Reconsidering the Convict Ships. In 2008, he was a senior Fulbright teaching fellow in Hungary.