Cullen entered St. Patrick's, Carlow College, in 1816, and, in 1820, he proceeded to the Pontifical Urban College in Rome, where his name is registered on the roll of students of 29 November 1820. At the close of a distinguished course of studies, he was selected to hold a public disputation in the halls of the Propaganda on 11 September 1828, in 224 theses from all theology and ecclesiastical history. The theological tournament was privileged in many ways, for Pope Leo XII, attended by his court, presided on the occasion, while no fewer than ten cardinals assisted at it, together with all the élite of ecclesiastical Rome. Vincenzo Pecci, the future Pope Leo XIII, was present at the disputation. Cullen graduated a doctor of divinity.