Maginnis studied at Cusack's Academy in Dublin and then at the South Kensington School in London where he won the Queen's Prize in mathematics in 1883.
Maginnis came to Boston in 1885 and entered the office of Edmund Wheelright, city architect of Boston, as a draftsman. He taught illustration at the Cowles Art School and drawing in the studio of the Boston Architectural Club. Also during this period he wrote a scathing article on the state of church building for Donahue's Magazine, denouncing in particular plaster cast statues. The article attracted much attention and Maginnis received his first commission for St. Patrick's Church in Whitinsville, Massachusetts, which he designed in the style of the 13th and 14th century Lombard churches. Other early churches by Maginnis in Massachusetts are St. Catherine, Somerville, and St. John's, North Cambridge.
In 1898 Maginnis entered private practice with Timothy Walsh and Matthew Sullivan and by 1906 had formed the firm of Maginnis and Walsh specializing in academic and ecclesiastical buildings. In 1907 he married Amy Brooks.
Other projects by Maginnis include building for Boston College, done in English Tudor style; Trinity College Chapel, Washington, D.C.; the library, dining hall, and chapel at Holy Cross College, Worchester, Massachusetts; Carmelite Convent, Santa Clara, California; the Law Library at the University of Notre Dame, South Bend, Indiana; the high altar and exterior bronze doors for St. Patrick's Cathedral, New York City; the National Shrine of the Immaculate Conception, Washington, D.C.; and the Cathedral of Mary Our Queen, Baltimore, Maryland.
Magginis served on the Massachusetts State Art Commission (1911-1920) and on the Boston Art Commission (1907-17). He won numerous awards, and authored books and articles in architectural magazines. He qualified for membership in the National Academy with a photograph of Trinity College Chapel, Washington, D.C.
