Lexington is a town located in Middlesex County, Massachusetts, United States. The population was 31,394 at the 2010 census, in nearly 11,100 households. Settled in 1642, this town is prominent for being the site of the first shot of the American Revolutionary War, in the Battle of Lexington on April 19, 1775, colloquially known as the "Shot heard round the world" when news spread about the revolution.