Fort McKinley (Great Diamond Island, Casco Bay, Maine)

Great Diamond Island, Casco Bay · Maine · Coastal Defense / World War I and World War II

Quick BriefFort McKinley operated as a United States Army coastal defense fort on Great Diamond Island from 1873 to 1947. Construction on the main installation began in 1897 and was complete by 1906, with nine gun batteries and facilities supporting an underwater minefield to defend against minesweepers. After closure, it was transferred to the Navy, which sold the site to private interests in 1961.

Endicott SystemCoastal defense

History & Significance

Following the Endicott Board's 1885 recommendation for comprehensive coastal fortifications, Fort McKinley was constructed beginning in 1897 and completed by 1906. The island was transformed from an artists' retreat visited by literary figures like Longfellow and Stowe into a military base protecting the deep-water Portland harbor.

By 1906, nine gun batteries were operational, including protective facilities for an underwater minefield, with original barracks and administration buildings built 1902–1906. Both Fort McKinley and its sub-post Fort Lyon on Cow Island were part of the Coast Defenses of Portland, renamed Harbor Defenses of Portland in 1925.

During World War I, Coast Artillery Corps and Maine National Guard troops manned the forts; after 1917 several guns were removed and shipped to the Western Front in France. In 1940–1941, the fort expanded to accommodate drafts and draftees; by 1943 most guns had been removed except a few 6-inch and 3-inch pieces.

With reduced threat by 1944, coast defense operations were drawn down, and after the war the Army withdrew and the properties were sold or transferred. The Fort McKinley Historic District was listed on the National Register of Historic Places in 1985.