Cornelius Van Ness attended Washington Seminary, and in 1800 he began to study at his brother William's law office in New York City at the same time as Martin Van Buren. Van Ness was admitted to the bar four years later, and moved to St. Albans, Vermont in 1806. He relocated to Burlington, Vermont in 1809 when he was appointed United States district attorney for the district of Vermont. He was made collector of customs for the district of Vermont in 1813 and in 1816 President James Madison named Van Ness one of the federal commissioners who negotiated with commissioners from Great Britain to settle the northeastern boundary between the United States and Canada.