Marlborough is a city in Middlesex County, Massachusetts, United States. The population was 38,499 at the 2010 census. Marlborough became a prosperous industrial town in the 19th century and made the transition to high technology industry in the late 20th century after the construction of the Massachusetts Turnpike.Marlborough was declared a town in 1660. It was incorporated as a city in 1890 when it changed its Municipal charter from a New England town meeting system to a Mayor–council government.