Cull's research and teaching interests are broad and inter-disciplinary, and focus on public diplomacy, the role of advocacy, culture, exchange, broadcasting, and public opinion research in foreign policy. Cull has also worked more broadly on the history of propaganda, film, television and radio history and the role of mass media as a source for historical study. He is best known for detailed historical studies of the institutions behind public diplomacy and for emphasizing the importance of "listening" as a pre-condition for successful public diplomacy. He coined the term reputational security for a category of enhanced security that comes to an international actor when they are well thought of by external audiences.
Cull is the co-editor of Propaganda and Mass Persuasion: A Historical Encyclopedia, 1500–present (2003), which was one of Book List magazine's official reference books of the year, and Alambrista and the U.S.-Mexico Border: Film, Music, and Stories of Undocumented Immigrants (2004; with David L. Carrasco).
With James Chapman, he has co-authored Projecting Empire: Imperialism and Popular Cinema (I.B. Tauris, 2009) and Projecting Tomorrow: Science Fiction and Popular Cinema (I.B. Tauris, 2013).
His most recent single authored work is Public Diplomacy: Foundations for Global Engagement in the Digital Age (Polity, 2019).
His most recent edited works are with Nancy SnowRoutledge Handbook of Public Diplomacy, 2nd edition (Routledge, 2020) and Michael HawesCanada's Public Diplomacy (Palgrave, 2020).