His priestly formation was informal but solid. He did not attend any seminary, although the Coulcil of Trent had established several of them in 1563. The clergy of his day were often rather ignorant, but Calasanz finished law school and received his doctorate.

He was ordained on December 17, 1583, in Sanahuja and held various offices in the Catalan region. His priestly work had many facets and was intense. Early in his priesthood, Calasanz showed his concern for the poor and disadvantaged by establishing a foundation in Claverol that distributed food to the destitute each year.