Cumberland County is a county in the U.S. state of Maine. As of the 2010 census, the population was 281,674, making it the most populous county in Maine. Its county seat is Portland. Cumberland County was founded in 1760 from a portion of York County, Massachusetts and named for William, Duke of Cumberland, a son of King George II. Cumberland County has the deepest and second largest body of water in the state, Sebago Lake, which supplies tap water to most of the county. The county is the state's economic and industrial center, having the resources of the Port of Portland, the Maine Mall, and having corporate headquarters of major companies such as Fairchild Semiconductor, IDEXX Laboratories, Unum, and TD Bank. Cumberland County is part of the Portland–South Portland, ME Metropolitan Statistical Area.