[1] On 24 July 1877 he was appointed Titular Archbishop of Heliopolis in Phoenicia by Pope Pius IX, receiving his episcopal consecration on the following 12 August from Cardinal Alessandro Franchi in Rome.

Mocenni was later named Apostolic Delegate to Ecuador, Peru, Nueva Granada, Venezuela, Guatemala, Costa Rica, Honduras, and Nicaragua on 14 August that year.

[1] Seven months later, on 18 October 1882, Mocenni entered the service of the Roman Curia upon being appointed Substitute, or deputy, of the Vatican Secretariat of State.

Pope Leo XIII created him Cardinal Priest of San Bartolomeo all'Isola in the consistory of 16 January 1893.

After lying in state in the church of Santa Maria in Traspontina, he was buried in the chapel of the Sacred Congregation of Propaganda Fide in the Campo Verano cemetery.