 Witherell was a member of the Vermont House of Representatives from 1798 to 1802.He was associate county judge from 1801 to 1803, judge of Rutland County from 1803 to 1806, and an executive councilor from 1802 to 1806.He was elected as a Democratic-Republican to the Tenth Congress, serving from March 4, 1807 until May 1, 1808, when he resigned to accept an appointment by President Thomas Jefferson as one of the Judges of the Supreme Court for the Territory of Michigan.