After serving in the militia, he joined the Continental army - serving throughout the war in the Quartermaster's Department. During that time he was stationed in Albany where he employed a number of clerks, assistants, and other field personnel.

While in Albany, he boarded at the riverside home of James Bloodgood. In 1779, the tax list showed him living there with personal property valued at $2,000 - one of the highest assessments in the city. Later, he qualified for a "bounty right" as a member of the city regiment of the Albany County Militia. During that time, he acquired some Albany property. But in 1788, his lot was taken back by the city government for failure to pay back rents.

Lewis married Gertrude Livingston in 1779. After the war, they lived in Rhinebeck and then in Hyde Park.

He was a member of the New york State Assembly (1789-92); Attorney General (1791-92); justice and chief justice of the New York State Supreme Court (1792-1801). He later served in the New York State Senate.

He was governor of New York from 1804-1807 and maintained an Albany residence during that time.

During the War of 1812, he served as Quartermaster General of the American army.