[1] His doctoral dissertation was published in London in 1914 by Longmans, Green and Company under the title The English Catholic Refugees on the Continent, 1558–1795, volume 1.
Guilday intended to follow this up with a second volume on the Irish Colleges on the Continent, but research for this was made impossible by the outbreak of World War I.
[1] Instead, in 1914, Guilday began teaching at Catholic University of America, Rector Thomas J. Shahan having asked Edmond Francis Prendergast, Archbishop of Philadelphia, to release him from diocesan duties so that he could join the faculty.
[2] His writings established him as the period's leading scholar in Catholic Church History, with appointment as full professor in 1923.
[2] He was relieved of teaching duties in 1941, and intended to use his time to produce a study of John Hughes, Archbishop of New York, but was prevented by poor health.