 Initially studying with the Sisters of the Immaculate Conception in Ivrea, he then attended seminaries in Seveso, Monza, and Milan (where he obtained a doctorate in theology in 1926), and received a doctorate in letters from the Catholic University of Milan in 1932.Receiving the clerical tonsure on 26 May 1923, Colombo was eventually ordained to the priesthood by Cardinal Eugenio Tosi, OSSCA, on 29 May 1926 in the Cathedral of Milan.He was then made Professor of Letters at the Seveso seminary in October of that same year.