Pope Pius IX was head of the Roman Catholic Church from 16 June 1846 until his death on 7 February 1878, making him the longest-reigning elected pope in history.

Born Giovanni Maria Mastai-Ferretti on 13 May 1792 in Italy, he was ordained a priest in 1819, became Archbishop of Spoleto in 1827 and then Cardinal-Priest of Santi Marcellino e Pietro in 1840. He was known as a liberal and advocated a simple life of poverty. After the death of Gregory XVI in 1846, Cardinal Mastai was elected Pope Pius IX. During his 32-year reign, Pope Pius IX issued 38 encyclicals on topics including the Immaculate Conception of Mary and the Church in various geographical regions; he also convened the First Vatican Council, which defined papal infallibility. During his papacy, the Papal States over which he was sovereign were captured and taken over by the Italian government during a series of revolutionay conflicts beginning in the 1840s. In 1870, the Italian government captured Rome and the pope retreated to the Apostolic Palace. He retained control over the Vatican City as sovereign, and became known as the "Prisoner of the Vatican."

Pope Pius IX died on 7 February 1878. He was beatified on 3 September 2000 by Pope John Paul II, after a history of opposition by the Italian government to his beatification and protests from some Jewish and Christian communities.