Demarest (third from right), in 1904, with members of the New Brunswick Seminary faculty. His predecessor as Professor of Ecclesiastical History and Church Government, Samuel Merrill Woodbridge (third from left) is seated in robes next to him.
Having been educated at the Rutgers Grammar School (now the Rutgers Preparatory School), Demarest graduated with high honors from Rutgers College with a baccalaureate degree in 1883. From 1883 to 1886, Demarest taught at the Rutgers Preparatory School. In 1888 he graduated from the New Brunswick Theological Seminary, and that same year was ordained to the ministry of the Dutch Reformed Church, where he served until 1901 at which time he returned to the Seminary, and was appointed Professor of Ecclesiastical History and Church Government to replace retiring professor Samuel Merrill Woodbridge (1819–1905). In 1905, Demarest was named as acting President of the College, and was elected by the Trustees to succeed Austin Scott as President in early 1906. During his tenure, the New Jersey College for Women (now Douglass College) was established in 1918, through private donors and increased appropriation from the State of New Jersey, new facilities were constructed for instruction in Engineering, Chemistry, Entomology, and Ceramics and dormitories were built to accommodate the increased enrollment. Following his resignation as President in 1924, Demarest served for ten years as president of the New Brunswick Theological Seminary and remained active in the affairs of the University. In 1924, he published History of Rutgers College. On the College Avenue section of the college's New Brunswick Campus, a dormitory was built in honor of Demarest. Aptly named Demarest Hall, it is located across the street from the Rutgers Student Center.