Leonard was born 19 December 1676, the son of Domenico Casanova and Anna Maria Benza. He was named Paul Jerome Casanova. His father was a ship captain whose family lived in Port Maurice on the north-western coast of Italy.

He joined the Friars Minor in 1697. On 2 October 1697, he received the habit and took the name Brother Leonard. After making his novitiate at Ponticelli in the Sabine mountains, he completed his studies at St. Bonaventura on the Palatine at Rome.

As Leonard contracted a bleeding ulcer after ordination, he was sent to his hometown where there was a monastery of the Franciscan Observants. After four years he was restored to health and began to preach in Porto Maurizio.

In 1710 he founded the monastery of Icontro, on a peak in the mountains about four miles from Florence.

Alphonsus Liguori called Leonard "the great missionary of the 18th century". He attracted large crowds and was invited to visit and preach in many places. Leonard spent over forty years preaching retreats, Lenten sermons and parish missions throughout Italy. His missions lasted 15 to 18 days, and he often stayed an additional week to hear confessions.

Pope Clement XII and Pope Benedict XIV called him to Rome; the latter especially held him in high esteem both as a preacher and as a propagandist and exacted a promise that he would come to Rome to die. Pope Benedict XIV appointed him to several complex diplomatic assignments. In Genoa and Corsica, in Lucca and Spoleto the citizens expected a bejewelled cardinal to represent the intentions of the pope. Instead, they were confronted by a humble, shoeless, muddy friar to confound their hostility and pride.

Leonard founded many pious societies and confraternities and exerted himself to spread devotion to the Sacred Heart of Jesus, and the perpetual adoration of the Blessed Sacrament. He also began to insist that the concept of the Immaculate Conception of Mary be defined as a dogma of the faith.

As a Franciscan priest, he preached the Way of the Cross at missions for forty-three years and reportedly set up stations in 571 locations throughout Italy, including the Colosseum in Rome.

In November 1751 Pope Benedict XIV called him to Rome. By that time, the strain of his missionary labors and his mortifications had completely exhausted his body. He arrived at his beloved monastery of St. Bonaventura on the Palatine on the evening of 26 November 1751 and died on the same night at the age of seventy-four.

Pope Pius VI pronounced his beatification on 19 June 1796, and Pope Pius IX his canonization on 29 June 1867.

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3. Saint Basolus
4. Saint Bellinus
5. Saint Faustus