In the papal conclave held from 14 to 16 June 1846, Giovanni Maria Mastai Ferretti, Bishop of Imola, was elected on the fourth ballot to succeed the recently deceased Gregory XVI as pope.

The conclave was the last to elect a ruler of the Papal States, the extensive lands around Rome and Northern Italy which the Catholic Church governed until 1870.

[4] The conservatives wished to see a continuation of papal absolutism in the governance of the Papal States, a continuation of the hardline policies of Pope Gregory XVI and his right-wing Secretary of State, Luigi Lambruschini, while the liberals wished for some measure of moderate reform and favored two candidates in Tommaso Pasquale Gizzi and Giovanni Maria Mastai Ferretti.

[6] Despite not having campaigned for the papacy, Cardinal Mastai Ferretti was perceived to be "a glamorous candidate, ardent, emotional with a gift for friendship and a track-record of generosity even towards anti-Clericals and Carbonari.

He was a patriot, known to be critical of Gregory XVI "[5] Faced with deadlock and persuaded by Bernetti to keep Lambruschini from being elected pope, liberals and moderates decided to cast their votes for Mastai Ferretti in a move that contradicted the general mood throughout Europe.

In fact, celebrations began to take place in his hometown, and his personal staff, following a long-standing tradition, burned his cardinalitial vestments.

On the following morning, the senior Cardinal-Deacon, Tommaso Riario Sforza, announced the election of Mastai-Ferretti before a crowd of faithful Catholics.

Emperor Ferdinand of Austria had charged Cardinal Karl Kajetan Gaisruck, the Archbishop of Milan (then part of the empire's territory), with vetoing the liberal Ferretti.