After completing the humanities in the Gymnasium of Straubing in 1821, Stadler entered the University of Landshut, where, in addition to the philosophical and theological studies prescribed for candidates to the priesthood, he devoted much of his time to the study of Middle Eastern and modern languages.
After being ordained priest by Bishop Johann Michael Sailer at Ratisbon 22 June 1827, he was occupied a few months in parochial work at the little village of Otzing in lower Bavaria, whereupon he continued his theological studies at the Ducal Georgianum in Munich in November, 1828, and obtained the doctorate in Theology in 1829.
Besides having a mastery of German, French, Italian, and English among the modern languages, he knew Latin, Greek, Hebrew, Syriac, Arabian, Persian, Sanskrit, and in his later years he studied also Spanish and Polish.
Stadler was best known as the author of Vollständiges Heiligen-Lexikon oder Lebensgeschichten aller Heiligen, Seligen u.a.
Kirche gefeiert oder sonst geehrt wird (Augsburg, 1858–82), an alphabetical collection of lives of Catholic saints from all over the world and from many different eras.