Remains of the Roman settlement of Caesaromagus have been discovered in the locality. In 1227 Chelmsford town became the regular seat of the county judicial gatherings known as assizes and quarter sessions. It remains the county town (seat) of Essex. The ecclesiastical diocese of Chelmsford, established in 1914, coincides with the county of Essex, and in 1951 the 15th-century parish church of St. Mary was designated as the diocese's cathedral. Chelmsford town has a county museum, an agricultural market, and a grain exchange. Light engineering, especially electronics, and administration provide the main employment, but some agriculturally based industries survive. From the local premises of Marconi's wireless telegraph company, the first wireless telegraph broadcasting service in the world was transmitted on Feb. 23, 1920.

The borough of Chelmsford encompasses the surrounding countryside within about 10 miles (16 km) of the town. Area borough, 132 square miles (342 square km). Pop. (2001) town, 99,962; borough, 157,053.

town (township), Middlesex county, northeastern Massachusetts, U.S. It lies 24 miles (39 km) north of Boston; the city of Lowell is adjacent to the northeast. Settled in 1633, it was named for Chelmsford, England, and incorporated in 1655. An iron foundry using local bog ore was built there in 1656. During the early 19th century, the production of lumber, textiles, and granite was important. Services, trade, and the production of electronics, computer equipment, and other light manufactures are now central to the economy.

In 1657 John Fiske (Fiske, John), an early pastor of the Chelmsford Unitarian Church, compiled the Chelmsford Catechism, the only extant copy of which is in the New York Public Library. Joseph Spalding of Chelmsford is said to have fired the first shot in the Battle of Bunker Hill (Bunker Hill, Battle of) (1775). Warren H. Manning State Forest is nearby. Area 23 square miles (60 square km). Pop. (1990) 32,383; (2000) 33,858.