Mazzuchelli arrived in the mid 1830s to what would later become Dubuque, Iowa. While there, he reorganized the parish and named it Saint Raphael's, which later became the Cathedral parish when the Dubuque Diocese was formed in 1837. He assisted Bishop Mathias Loras during the first few years after the founding of the Diocese and worked extensively in what would eventually become the Diocese of Madison, Wisconsin. There he founded over 30 parishes and designed and built over 20 church buildings, along with a number of civic buildings. Three of those parishes were named after the three Archangels: Saint Raphael's in Dubuque, Saint Michael's in Galena, Illinois, and Saint Gabriel's in Prairie du Chien, Wisconsin. In 1848, he founded St. Clara Academy ( now Dominican University of Illinois), a