founder of the Roman Catholic Ordo Clericorum Regularium Matris Dei (Clerks Regular of the Mother of God), whose members were commonly called Leonardini; the order was distinguished for learning and was originally devoted to combatting Protestantism and to promoting the Counter-Reformation.

Joining the now-extinct Apostolic Clerics of St. Jerome, John was ordained priest c. 1572 and became particularly active in charity. He soon attracted and trained some laymen and in 1574 founded his religious congregation in Lucca. In 1579 he formed the Confraternity of Christian Doctrine; his congregation of Clerks Regular received episcopal approval in 1583.

John's campaign against Protestantism, however, was so fervent that only with papal protection was he admitted into the Republic of Lucca, which opposed his policies for political reasons. He was aided by St. Philip Neri, the religious reformer, and St. Joseph Calasanz, whose order of the Pious Schools (Piarists) temporarily merged with the Leonardini. John's work attracted the support of Pope Clement VIII, who in 1595 officially approved John's congregation. Having begun four more religious orders amidst his reforming activities, John also cofounded (1603) a seminary to train priests for foreign missions. He died after contracting influenza while nursing epidemic victims. Sta. Maria in Campitelli, Rome, mother church of the Clerks Regular, contains his relics.