Thaddeus Mason Harris (July 7, 1768 - April 3, 1842) was a Harvard librarian, Unitarian minister and author. He was born in Charlestown, Massachusetts. After his father’s death while fighting on the colonial side in the American Revolutionary War, Harris was sent to live on a farm in Sterling, Massachusetts. Harris studied at Harvard University and graduated in 1787. Following his graduation, Harris spent a year as a school teacher in Worcester, Massachusetts, after which he was offered an appointment as secretary to George Washington. Unfortunately he contracted small-pox, and his lengthly recovery prevented him from taking the post. In 1791, Harris became the librarian at Harvard and was appointed as the minister of the First Unitarian Church on Meeting House Hill in Dorchester, Massachusetts, in 1793. In 1806, he was elected as a Fellow of the American Academy of Arts and Sciences and was also a founding member of the American Antiquarian Society in 1812. He served as corresponding secretary for the Antiquarian Society from 1812-1831, and as secretary of foreign correspondence from 1831-1832.