He was ordained to the priesthood by Bishop Vittorio Longo on 19 July 1964. He then taught canon law at his alma mater, the Theological Faculty of Southern Italy. From 1971 to 1978 he taught ecclesiastical law at the Pontifical Lateran University, while also serving as adviser for the Italian Catholic University Federation, the movement Seguimi, and the Union of Major Religious Superiors of Italy. He later became Rector of the Major Archiepiscopal Seminary of Naples and Regional Counselor of Catholic Action. On his return to Naples in 1978 he became rector of the major seminary until 1987, when he took up the post of dean of the S. Tommaso d'Aquino campus, where he had continued to teach.
On 26 March 2017 Pope Francis accepted his resignation as Vicar of Rome and Archpriest of St. John Lateran.[4] On 30 September Francis named him a member of the Supreme Tribunal of the Apostolic Signatura.[5]
Distributing condoms in schools in the context of combating AIDS trivializes sexuality and education and must be fought by concerned parents, according to Vallini. In an official statement, he responded to a decision by the civil authorities of the province of Rome to allow the distribution of condoms from vending machines in high schools (see religion and AIDS).[7][8]