He emigrated to Boston at age 18, trained as an architect and went on to form the firm Maginnis & Walsh, designing ecclesiastical and campus buildings across America.

In 1909, Maginnis & Walsh won the competition to build the new campus of Boston College in Chestnut Hill, Massachusetts.

In other parts of the country he designed the Basilica of the National Shrine of the Immaculate Conception in Washington, D.C., the Cathedral of Mary Our Queen in Baltimore and the interior of Emmanual Masqueray's Basilica of St Mary in Minneapolis as well as the sacristy and rectory for the Cathedral of St. Paul in Saint Paul.

Among his other designs are the chancel at Trinity Church in Boston's Copley Square and the high altar at St. Patrick's Cathedral, New York.

In 1948, the Institute presented him with the Gold Medal for "outstanding service to American architecture," the highest award in the profession.