Marlborough is a city in Middlesex County, Massachusetts, United States. The population was 41,793 at the 2020 census. Marlborough becam…Marlborough is a city in Middlesex County, Massachusetts, United States. The population was 41,793 at the 2020 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. It is part of the Worcester metropolitan area.