After studying in local seminaries, he was ordained a priest on 28 June 1957 by Archbishop Giovanni Battista Montini, the future Pope Paul VI.

[1] In 2006, giving the opening address at the decennial conference of the Italian Catholic Church, he challenged the traditional alliance of the hierarchy with the country's conservative parties and called for more flexibility in forming alliances in contrast to the preference of Cardinal Camillo Ruini, head of the Italian Episcopal Conference, for Silvio Berlusconi.

[5] In response to Pope Benedict's Summorum Pontificum of July 2007, Tettamanzi stated that the rules it presented for the celebration of the Mass in different forms did not apply to the Milan archdiocese since it uses the Ambrosian Rite rather than the Roman.

[6] In March 2009, as required upon reaching the age of 75, he submitted his resignation,[7] and Pope Benedict accepted it on the 54th anniversary of his ordination to the priesthood, 28 June 2011.

[11] In 2015, at the behest of Pope Francis, he produced a study of the feasibility of creating a Dicastery for the Laity, Family and Life for consideration by the Council of Cardinals.

[17][18] In 2008, in a letter addressed to Catholics who had divorced and remarried, he wrote:[19] The fact that these relationships are frequently lived with a sense of responsibility and with love among the couple and for the children is a reality that the Church and its pastors take into account.

"[19] In 2014, he published Il Vangelo della misericordia per le famiglie ferite (The Gospel of Mercy for Wounded Families) and said he anticipated allowing divorced and remarried Catholics to receive communion as long as there is no confusion about the Church's insistence on the indissolubility of marriage and there is "a restored commitment to Christian life through faithful paths that are true and serious.