Narrated tours of Diamond Cove are available from a group of resident volunteers.  Tours need to be booked in advance. If you are interested in arranging a tour, please email .

The summer hours of operation of the Diamond Cove Museum can be found here.

Just before the turn of the 20th century, Great Diamond Island was transformed from an artists’ retreat — visited by American literary greats such as Henry Wadsworth Longfellow and Harriet Beecher Stowe – to a military base. The island fort was built as part of the United States’ coastal defense system to protect Casco Bay and the deep-water Portland harbor. Construction started in 1891, when the nation was only vulnerable to attacks by sea. The fort was finished in 1906 after serving as a key military base during the Spanish-American War from 1898 to 1902. In 1902, the fort was named for President William McKinley, who was assassinated a year earlier. When it was complete, Fort McKinley’s barracks, officers’ quarters and NCO quarters could house nearly 1,000 soldiers. Among the 70 structures built at Fort McKinley between 1891 and 1943 were roughly three dozen tactical buildings, including mining casements, batteries, observation stations, telephone switchboard buildings, and storehouses. By the end of World War II, the development of air war tactics made the island fort obsolete, and it was abandoned.

After decades of neglect and decay, the fort has been restored to its original elegance. Now known as the community of Diamond Cove, the barracks and officers’ quarters have become gracious homes with slate roofs, wooden porches, grand staircases and fireplaces. The former quartermaster’s storehouse is a fine restaurant, the wagon shed is an art gallery, and the blacksmith shop a general store. Diamond Cove is listed in the National Register of Historic Places, and every effort is made to preserve its architectural integrity.

Diamond Cove is now a year-round community of historic homes and waterfront properties. The population quadruples in the summertime as the majority of homeowners make their way back to the island and visitors arrive to experience our island resort life. Outside the boundaries of Diamond Cove there is a sister community of Great Diamond Island homeowners. The state of Maine operates the dock on the south side of the island (GDI), and a second dock is privately operated in Diamond Cove (DC), on the north side. Both docks are used by Casco Bay Lines to ferry residents and visitors to and from Portland. The entire island sits within the Portland city limits.