New Haven County, ,established in 1666, consisted of Milford, New Haven, and Guilford. The town of Wallingford was established in 1670 just north of New Haven and formally added to New Haven County in 1671. In 1675, the town of Derby was established north of Milford. In 1686, the town of Waterbury was established, but was assigned as part of Hartford County. Waterbury was transferred to New Haven County in 1728. In 1722, most of northwestern Connecticut (except for the town of Litchfield) was placed under the jurisdiction of New Haven County. Eight years later, in 1730, the eastern half of northwestern Connecticut was transferred to the jurisdiction of Hartford County. By mid-1738, with the exception of the towns of New Milford, Sharon, and Salisbury, the entire territory of northwestern Connecticut was under Hartford County. Wikipedia