 He was the son of businessman and politician William Edward de Mille (1824–1873) and Margaret Blount Hoyt de Mille (1835–1911).He received his BA and AM degrees from Columbia College in 1875 and 1879.He had studied for the ministry before choosing instead to teach and eventually serve as vice-principal at the Lockwood Academy in Brooklyn and later teach for some semesters at the Columbia Grammar School in Manhattan.