Tonti was ordained to the priesthood on 21 December 1867, and then served as professor of theology and vice-rector of the Pontifical Urbanian Athenaeum De Propaganda Fide until 1879.

He became an official of the Sacred Congregation of Extraordinary Ecclesiastical Affairs in the Roman Curia before serving as auditor of the nunciature to France (1879–1882) and to Portugal (1882–1892).

On 11 July 1892, Tonti was appointed Apostolic Delegate to Santo Domingo, Haiti, and Venezuela and Titular Archbishop of Samos by Pope Leo XIII.

He received his episcopal consecration on the following 25 July from Cardinal Vincenzo Vannutelli, with Bishops Michele dei Baroni Zezza Zaponetta and Augusto Berlucca serving as co-consecrators.

He was later named Prefect of the Sacred Congregation of Religious on 13 February 1917, also becoming a member of the Council for the Administration of the Wealth of the Apostolic See on 21 May of that same year.