 He was a judge of the Vermont Supreme Court in 1833 and 1834, and declined to be a candidate for renomination.Mattocks was a delegate to the State constitutional convention in 1836 He was elected as a Whig to the Twenty-seventh Congress (March 4, 1841 – March 3, 1843).In 1843, the major candidates for Governor of Vermont were Mattocks (Whig), Daniel Kellogg (Democrat), and Charles K. Williams (Liberty).