He became tutorial fellow in Latin (1890-1891) and was appointed instructor in Latin and Greek (1891-1902), and adjunct professor of classical philology (1902-1906). In 1906, he became a noted professor of classical philology at Barnard College, a women"s liberal arts college affiliated with Columbia University. Doctor Knapp used to say to his freshmen that he and they were alike travelers on the road of learning, and the space between was so small, in comparison with the length of the road, that it need be no hindrance to pleasant companionship thereon.
Throughout his long teaching life this was his characteristic attitude, and his students recognized that his passion for intellectual honesty and hard work were equaled by his kindness and enthusiasm.
Knapp was married to Theresa Shaw in 1889 in Manhattan, New New York Together, they had one son, Charles M. Knapp, born 21 September 1892.