He was accepted as a seminarian for the Archdiocese of Cincinnati by John Baptist Purcell, and sent to Mount St. Mary's University for studies.

[1] Quinlan's first assignment as a priest was in Piqua, Ohio, before serving as curate for future Archbishop of Philadelphia James Wood at St. Patrick's Church in Cincinnati.

[1] In his diocese he found twelve churches and fourteen schools for which he had only eight secular priests and he therefore brought from Ireland eleven young candidates for the priesthood.

After the battle of Shiloh, he hastened on a special train to the blood-stained battle-ground and ministered to the temporal and spiritual wants of North and South.

In April 1876, Bishop Quinlan invited the Benedictines from St. Vincent's Abbey, Pennsylvania to the diocese, and they settled at Cullman, Alabama.