Monroe County is a county located in the U.S. state of Michigan. As of the 2010 census, the population was 152,021. The largest city and county seat is Monroe. The county was established as the second county (after Wayne County) in the Michigan Territory in 1817 and was named for then-President James Monroe.Monroe County comprises the Monroe, MI Metropolitan Statistical Area and is included in the Detroit-Warren-Ann Arbor, MI Combined Statistical Area.