 He was ordained to the priesthood on 18 April 1937, and then served as vice-rector of the Piacenza seminary until 1939.Tonini furthered his studies at the Pontifical Lateran University in Rome, obtaining his degrees in civil and canon law, from 1939 to 1943.From 1943 to 1969, he did pastoral work in the Diocese of Piacenza and for many years he was the pastor of parish of Salsomaggiore, as well as serving as ecclesiastical assistant of the Federation of Catholic Italian University Students, director of the weekly "Il nuovo giornale", and professor and rector at its seminary.