[1] Ordained to the priesthood on 4 June 1887, he entered the Oblates of Saints Ambrose and Charles in 1889, after serving as a curate in Busto.

Tosi then taught at the Missionary House of the Oblates in Rho until 1909, when he was made vicar general of Rimini.

[2] 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.

He publicly denounced the playing of the opera The Martyrdom of San Sebastian, which Catholics were prohibited from seeing, at La Scala in 1926.