Middlesex County is located in the Commonwealth of Massachusetts, in the United States. As of the 2020 census, the population was 1,632,002, making it the 22nd most populous county in the United States, and the most populous county in both Massachusetts and New England. Middlesex County is one of two U.S. counties (along with Santa Clara County, CA) to be amongst the top 25 counties with the highest household income and the 25 most populated counties. As part of the 2010 national census, the Commonwealth's mean center of population for that year was geo-centered in Middlesex County, in the town of Natick at (42.272291°N 71.363370°W﻿ / 42.272291; -71.363370). (This is not to be confused with the  geographic center of Massachusetts, which is in Rutland, in neighboring Worcester County.) Middlesex County is included in the Census Bureau's Boston–Cambridge–Newton, MA–NH Metropolitan Statistical Area.
On July 11, 1997, the Massachusetts legislature voted to abolish the executive government of Middlesex County due primarily to the county's insolvency. Though Middlesex County continues to exist as a geographic boundary it is used primarily as district jurisdictions within the court system and for other administrative purposes, such as an indicator for elections. The National Weather Service weather alerts (such as severe thunderstorm warning) continue issuances based upon Massachusetts's counties.




As of the 2010 United States Census, there were 1,570,328 people residing in the county. The racial makeup of Middlesex County was:
In terms of ancestry groups that make up a plurality (greater than 25%) within Middlesex County are: 
The median income for a household in the county was $86,890 and the median income for a family was $98,452.

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