Giovanni Colombo studied at the University of Milan, the subjects philosophy and Catholic theology. He received on 25 May 1926 by Eugenio Cardinal Tosi the sacrament of Holy Orders, and then taught at the Seminary and at the University of Milan. From 1939 to 1953 he served as rain, the minor seminary in Milan, from 1953 to 1960, he provided the same task in the seminary.
1960 he was appointed by Pope John XXIII. Titular Bishop of Philippopolis in Arabia and Auxiliary Bishop of Milan. He received his episcopal consecration Cardinal Giovanni Battista Montini, the future Pope Paul VI .. 1963 Giovanni Colombo appointed Archbishop of Milan. In 1965, Pope Paul VI. as cardinal priest with the titular church of Santi Silvestro e Martino ai Monti in the College of Cardinals.
His resignation was accepted in 1979 by John Paul II. As of January 1980, he lived in the seminar Corso Venezia in Milan. Giovanni Colombo died on 20 May 1992 with 89 years in Milan and was buried in the local cathedral. At the funeral two days later, which was presided over by Cardinal Carlo Maria Martini, among others, Cardinal Vicar Camillo Ruini, Cardinal Giacomo Biffi and Marco Cardinal Cézanne were present.