Merrimack County was organized at Concord in 1823 and is named for the Merrimack River. It was formed by removing towns from northern Hillsborough and Rockingham Counties.
Merrimack County is a county in the U.S. state of New Hampshire which has two cities and twenty-five towns. As of the 2020 Census, the population was 153,808. Its county seat is Concord.
Merrimack County has a total area of 956 square miles (2,476 km2), of which 934 sq mi (2,419 km2) is land and 22 sq mi (57 km2) (2.31%) is water. It is the third-largest county in New Hampshire by land area. The highest point in Merrimack County is Mount Kearsarge, on the border of Warner and Wilmot, at 2,937 feet.