In 1801 Jefferson appointed Mansfield as professor at the newly founded United States Military Academy at West Point.

He fell into "bad company" and was expelled from college in January of his senior year for complicity in a theft of books from the Library and "other discreditable escapades".

[3] Apparently reforming, he may have taught in New Haven, perhaps at the Episcopal Trinity Church on the Green schools where he is listed as a "clerk of the vestry" appointed by Rev.

[7] In 1801 he had printed some scientific papers titled Essays Mathematical and Physical, which were brought to Jefferson's attention by Senator Abraham Baldwin.

[7] In 1801, the position had been offered to Andrew Ellicott by Jefferson, but he refused, because he was upset at slow pay for work he had done for the Federal Government.

"[10] He laid out baselines and Meridians astronomically, adapting principles of celestial navigation to surveying,[9] which Ellicott and others had used to extend the Mason–Dixon line decades earlier.

Mansfield returned to West Point in 1814 as professor of mathematics and natural and experimental philosophy, continuing in this position until his retirement in 1828.