The Jefferson County Courthouse is located at 163 Arsenal Street, Watertown NY 13601; phone: 315‑785‑3000.
Established in 1805, the county was named for the 3rd U.S. president, Thomas Jefferson. The early 19th century county included portions of towns which eventually became part of Oneida County. After 2 more boundary adjustments, the county achieved its present size in 1813. There was spirited competition among several towns vying to be picked as the county seat. But Watertown, already economically substantial in the area, won the honor. The first white settlers to form permanent residence in the county were Judge Noadiah Hubbard and Lyman Ellis, arriving in 1797. By 1807, 11 towns had incorporated, and the county population was just shy of 2,000. Over the next 50 years it grew to about 70,000. 
Jefferson County is home to Fort Drum — its' history dates to the Camp Hughes-New York National Guard encampment established across the Black River, opposite Felts Mills in 1907. The first week of that September, nearly 2,500 Guardsmen trained on 800 acres of land 'on the pine plains.' In 1908, Brigadier General Frederick Dent Grant, son of General Ulysses S. Grant, arrived to train with 10,000 Soldiers. He found Pine Plains to be an ideal place to train troops. The purchase of additional land in 1909 established the Pine Camp as a permanent National Guard facility. Semi-permanent buildings were constructed in the early 1920's. The First US Army maneuvers of 1935, the largest peace-time maneuvers in our nation's history at that point, involved five regular Army and National Guard Divisions, in a 78-square-mile area adjacent to Pine Camp. The Pine Camp training area grew by 9,000 acres.