Bishop Carroll came from Dubuque, Iowa, where he served as president of St. Joseph's College (now Loras College). Bishop Carroll was a builder and an educator. He vastly increased the number of Catholic elementary and secondary schools in the diocese. In addition he built Mount St. Charles College (now Carroll College) with two purposes in mind: "to provide men of high intellectual training and to provide a native priesthood." Today, the vast majority of secular priests serving in the Diocese of Helena are graduates of Carroll College. The college also can count two bishops among its alumni: Archbishop Raymond G. Hunthausen, formerly Bishop of Helena, and Archbishop of Seattle, and Bishop Bernard J. Topel of Spokane, now deceased.
Bishop Carroll also succeeded in introducing numerous religious orders into the diocese to operate schools and hospitals. The Irish Christian Brothers and the Premonstratensian Fathers also came to the diocese under the auspices of Bishop Carroll.
Bishop Carroll is also remembered for his building of the magnificent Cathedral of St. Helena. Modeled after the Votive Church of Vienna, this Cathedral in the See City is one of the most beautiful in the United States. Many of the contributors to its building were people of prominence in Montana and American history: Thomas Cruse, discoverer of the greatest gold mine in American history; T.C. Power, early Montana "merchant prince"; and, Senator Thomas J. Walsh, investigator of the famous Teapot Dome scandal.