After passing the bar exam, he returned to St. John’s to teach Latin and Greek, but after his third year there, he left for a year to study medicine with Dr. A.
R. Mackey at New York University Medical School, but never continued his medical studies after that year. In 1878, Moore returned to St.
John’s as a professor of Greek and Latin and remained there until his death in 1936. In 1886, he wrote his first book, The Romans in Spain: A Brief Account of Spanish History from the Earliest Times to the End of the Umayyad Period (1887), which was awarded best history book of 1888 by the American Historical Association. His next book was a biography titled A Story of New York City: The Life and Times of Benjamin Franklin (1893) which won him high praise from the New York Times Book Review.
In 1896, Moore published a work describing how Jesus Christ saved mankind from sin called The Atonement: Its Cause and Its Purpose (1896). In 1899 he published The Life Story of Jesus Christ (1900), which was awarded best religious book of the year by the American Book Company. In 1914, Moore published a book describing how Jesus Christ created man called The Creative Will: Its Origin and Development (1914).
In 1917 he published a biography titled The Lives of Robert E. Lee: Leader in War and Peace (1917), which won him high praise from The American News Magazine. That same year he also published a work titled Darwin: Founder of Evolution: How He Discovered It and Proved It (1917).
In 1921 he published a work titled Charles Sanders Peirce: His Life and Work (1921) which won him high praise from Scientific American Magazine. In 1925 he published another book titled Jefferson Davis: His Rise and Fall (1925), which won him high praise from The Literary Digest Magazine.