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 most populous county in both Massachusetts and New England and the 22nd most populous county in the United States.

Prior to 1960, Middlesex County was a Republican Party stronghold, backing only two Democratic Party presidential candidates from 1876 to 1956. The 1960 election started a reverse trend, with the county becoming a Democratic stronghold. This has been even more apparent in recent years, with George H. W. Bush in 1988 being the last Republican presidential candidate to manage forty percent of the county's votes and Mitt Romney in 2012 being the last Republican presidential candidate to manage even thirty percent of the vote. In 2020, Joe Biden won 71% of the vote, the highest percent for any presidential candidate since 1964.