On February 9, 1791, Chipman met with President George Washington to notify him officially of Vermont's decision to apply for admission to the Union as the 14th state. New York had long objected to the existence of the government of Vermont on the grounds that Vermont was part of New York, a position that dated back to a pre-Revolutionary War dispute between the colonial governors of New York and New Hampshire over the right to sell Vermont land grants. In 1790, New York agreed to give up its claim provided only that an agreement on the boundary between Vermont and New York could be concluded and that Congress would admit Vermont to the Union. Vermont's negotiators insisted on also settling the real-estate disputes rather than leaving those to be decided later in a federal court. Vermont paid $30,000 to settle the claims. On February 18, 1791, Congress decided to admit Vermont to the Union, effective March 4, 1791.
Following his resignation from the federal bench, Chipman resumed private practice in Tinmouth from 1793 to 1796. He served as Chief Justice of the Supreme Court of Vermont from 1796 to 1798.
Congressional service
Chipman was elected as a Federalist from Vermont to the United States Senate to fill the vacancy caused by the resignation of United States Senator Isaac Tichenor and served from October 17, 1797, until March 3, 1803. He was an unsuccessful candidate for reelection.
Later career
Following his departure from Congress, Chipman resumed private practice in Tinmouth from 1804 to 1806, from 1809 to 1810, from 1811 to 1813, and from 1817 to 1843. He was a member of the Vermont House of Representatives from 1806 to 1809, and in 1811. He was a member of the Vermont Council of Censors in 1813. He was Chief Justice of the Supreme Court of Vermont from 1813 to 1815. He was a Professor of law at Middlebury College starting in 1816.
Death
Chipman died on February 17, 1843, in Tinmouth. He was interred in Tinmouth Cemetery.
Family
Chipman was the brother of Daniel Chipman, a United States Representative from Vermont, the grandfather of John Logan Chipman, a United States Representative from Michigan, In 1781, Chipman married Sarah Hill, they had six children, including Henry C. Chipman. Another son, Jeffrey Chipman, was a Justice of the Peace in Canandaigua, New York in the 1820s, and was the jurist from whom those attempting to prevent William Morgan from publishing a book opposing Freemasonry obtained an arrest warrant for Morgan, which eventually led to Morgan's disappearance and presumed death and the founding of the Anti-Masonic Party. Chipman was the grandfather of John W. Brownson, a member of the New York State Senate. Brownson was the son of Dr. John Brownson and Nathaniel Chipman's daughter Laura.