Chelmsford, town and borough (district), administrative and historic county of Essex, England, lying in the valley of the River Chelmer northeast of Greater London in south-central Essex. Chelmsford town is the seat of the administrative county.
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, and 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. From the local premises of Guglielmo Marconi’s wireless telegraph company, the first wireless telegraph broadcasting service in the world was transmitted on February 23, 1920.
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. The borough of Chelmsford encompasses the surrounding countryside within about 10 miles (16 km) of the town. Area borough, 131 square miles (339 square km). Pop. (2001) town, 99,962; borough, 157,072; (2011) town, 110,507; borough, 168,310.