Hillsborough County is the most populous county in the U.S. state of New Hampshire. Its county seats are Manchester and Nashua. Hillsborough is northern New England’s most populous county as well as its most densely populated. Hillsborough County comprises the Manchester-Nashua, NH Metropolitan Statistical Area, which is also included in the Boston-Worcester-Providence, MA-RI-NH-CT Combined Statistical Area.
Hillsborough was one of the five original counties identified for New Hampshire in 1769, and was named for Wills Hill, the Earl of Hillsborough who was British Secretary of State for the Colonies at the time. The county was organized at Amherst on March 19, 1771. In 1823 a number of towns were removed to become part of Merrimack County. Over several years ending in 1869, county administrative functions were moved from Amherst first to Milford in 1866 then to the current seats of Manchester and Nashua.