In the mid-1880s, Croatian Bishop Josip Juraj Strossmayer began a movement which focused on unity and consecration of the cathedral of Đakovo and Srijem, a movement in which Leopold took interest in. On September 20, 1890, Leopold was ordained to the presbyterate at Venice at the age of 24.
Refusing to renounce his Croatian nationality during World War I, Leopold was forced to go to southern Italy. All this time Leopold held a hope that he would be able to return to his homeland and preach among his people, a feat that would be inhibited by his disabilities. On top of his physical deformities, he also suffered from stomach ailments, poor eyesight, and arthritis. Unsurprisingly, the Capuchin ministers declined these attempts due to his health.
While in Italy, Leopold's main vocation was confessions, which he did for 34 years. The Capuchin brothers often criticized Leopold for his approach to confession, calling him too lenient and compassionate. Leopold's compassion showed that he was more understanding and sympathetic to the people that came to him, and would treat them with great sensitivity. He was an outspoken on issues with children, and being pro-life and especially fond of expectant mothers and young children. He did great work in setting up orphanages for children without parents.
Leopold also had a deep devotion to the Virgin Mary who he referred to as "my holy boss". He was known to pray the rosary quite often, and celebrated the eucharist daily at the side altar in the Little Office of the Virgin Mary. He would then visit the sick in nursing homes, hospitals and homes all over Padua. He visited the Capuchin infirmary to comfort the sick friars, giving them words of advice and reminding them to have faith.
Leopold suffererd from esophagus cancer, which would ultimately lead to his death at age 76. On July 30, 1942, while preparing for the liturgy, he collapsed on the floor. He was then brought to his cell, where he was given the last rites. Friars that had gathered at his bed sang "Salve Regina," and when they got to the words, "O clement, O loving, O sweet Virgin Mary," Leopold died.
During the bombing of World War II the church and part of the friary where Leopold lived were demolished, but Leopold's cell and confessional were left unharmed. Leopold had predicted this before his death, saying, "The church and the friary will be hit by the bombs, but not this little cell. Here God exercised so much mercy for people, it must remain as a monument to God's goodness." Paul VI beatified Leopold on May 2, 1976. He was canonized by John Paul II during the Synod of Bishops on October 16, 1983. Leopold is hailed as the "Apostle of Unity."