In 1780 he became an associate professor of theology and philosophy at the University of Wittenberg, where he served as rector in 1790–91. In 1792 he was appointed Oberhofprediger (first preacher) to the Saxon court in Dresden.
Reinhard was one of the more influential Protestant ministers of his era, and was an important representative of "enlightened theological supernaturalism". He was not opposed to contemporary rationalist thought, yet at the same time stressed the importance of divine supremacy and Biblical authority. In his sermons and lectures he attempted to establish the "truth of Lutheranism" by rational means.
Reinhard was a prominent figure in the Leben-Jesu-Forschung (Quest for the Historical Jesus) movement, a concept initiated by Hermann Samuel Reimarus (1694-1768). He also exerted considerable influence upon the German educational system.