He studied law in
Norwich, was admitted to the bar in 1811 and practiced in
Rensselaer and Saratoga counties. Originally a member of the
Federalist Party, he served in local and judicial office, including justice of the peace, inspector of the common schools, Master in Chancery and Saratoga County Surrogate. In 1816, he moved to the area in the Town of
Pomfret which later became the Village of
Dunkirk. He was
Surrogate of Chautauqua County from 1819 to 1821, and
District Attorney from 1818 to 1826. Garnsey was elected as an
Adams candidate to the
19th and
20th United States Congresses, holding office from March 4, 1825, to March 3, 1829. In 1828, he ran unsuccessfully for re-election as an independent candidate with the support of the
Anti-Masonic Party, even though he had been a Mason himself. During the 1828 campaign, when Anti-Masons were considering whether to support
John Quincy Adams for reelection as president, Garnsey wrote a letter to Adams to ask whether he was a Mason. Adams replied that he was not a Mason, but had known several individuals who were, including
George Washington, and had no negative comments about them. Garnsey's letter and Adams' reply were made public in Anti-Masonic newspapers. Because Adams' opponent
Andrew Jackson was a Mason, while Adams was not, Anti-Masons supported Adams. In 1831, Garnsey moved to
Battle Creek, Michigan. He was Postmaster, and Government Superintendent of Public Works near
Detroit and
Ypsilanti. He served with
Winfield Scott in the
Black Hawk War in 1832. Garnsey became a
Whig when the party was founded in the 1830s, and he was a supporter of the presidential candidacies of
Henry Clay and
William Henry Harrison. He later moved to
Rock Island, Illinois. On March 22, 1841, he was appointed by Harrison as Receiver of Public Moneys at the Land Office in
Dixon, Illinois, and served until removed by President
John Tyler on August 25, 1843. When Harrison died in 1841 after only a few weeks in office, Garnsey was one of the official pallbearers at Harrison's funeral. ==Death and burial==