Biffi was born in Milan and studied at the seminaries of the Archdiocese of Milan. He was ordained to the priesthood by Cardinal Alfredo Ildefonso Schuster, O.S.B., Archbishop of Milan, on 23 December 1950. From 1951 to 1960, Biffi taught dogmatic theology at the Seminary of Milan, also publishing numerous works on theology, catechetics and meditation. He received a doctorate in theology from the Faculty of Theology at Venegono in 1955; his thesis was entitled: "La colpa e la libertà nell'odierna condizione umana".