 After becoming Bishop of Andria on 22 March 1917, he served as Apostolic Administrator of Squillace from 10 August 1917 to February 1918.Pope Pius XI named Tosi to succeed him as Archbishop of Milan on 7 March 1922 and created him Cardinal-Priest of "Ss.Silvestro e Martino ai Monti" in the consistory of 11 December the same year.