Great Diamond Island was initially an artists’ retreat community, but was converted to a military base at the time of the Spanish-American War to defend Casco Bay. Fort McKinley was constructed between 1891 and 1907 and remained in service until the end of World War II in 1945.
After 30 years of being idle, the former base site—which encompasses the middle of Great Diamond—was converted into a gated resort community called Diamond Cove. Condos in the converted barracks and officers’ quarters and private homesites are primarily owned by seasonal visitors. The majority of the year-round population lives on either end of the island.
- Year-round population, 2010 U.S. Census: 91
- Summer population estimate: 200
- Acres: 416
- Mainland ferry terminal: Portland
- Estimated distance between mainland and island ferry docks:
- Little Diamond: 2.5 miles
- Great Diamond: 3 miles
- Ferry service: Casco Bay Lines
- School: None, students commute to Portland Schools
- Form of town government: Part of the City of Portland