Discover endless possibilities at MGM National Harbor. WASHINGTON — MGM National Harbor opened in Prince George's County, Maryland, on Thursday, Dec. jpg, Governor speaks at The MGM Grand Opening by Steve Kwak at MGM National Harbor Hotel Lobby ; Date, 8 December . The MGM will be the 6th casino in Maryland, joining the likes of Horsehoe Casino, Maryland LIVE! and Perryville Races and Slots. The $ billion casino resort will debut just in time. MGM National Harbor will have its grand opening Dec. Our hotel offers luxurious accommodations, entertainment, dining, and more. It's a casino. The $ billion MGM National Harbor, Maryland's sixth and most expensive casino, opened Thursday with Las Vegas-style flair and the company. When it comes to money made from gambling, MGM National Harbor has become dominant among Maryland's six casinos. 8, The story resort and casino includes. DescriptionMGM Grand Opening (). 8, officials announced Monday. The new casino on the banks of the Potomac and in the shadow of the nation's capital filled to overflowing on its first full day of. That's what folks are saying about Prince George's County's new $ billion resort and casino, MGM National Harbor, which reached full capacity.