A 14-acre purchase near Portland Head Light in 1872 served to establish a sub-post to Fort Preble located at Spring Point.

Infrastructure included an electrical substation, a bunkered telephone switchboard, and pumps and underground storage tanks for gasoline and fuel oil.

[5] A history of the Coast Artillery in World War I states that none of the regiments in France equipped with 6-inch guns completed training in time to see action before the Armistice.

[4] A plaque next to one of the fort's remaining buildings states that it housed towed 155 mm guns following World War I.

Circular concrete platforms called "Panama mounts" were constructed at Fort Baldwin in Phippsburg, Maine and at Biddeford Pool to allow more effective use of these guns.

On 23 October 1939, the 3rd Battalion sailed from Portland, ME, on the USAT Chateau Thierry as part of the 18th Infantry Brigade force sent to reinforce the Panama Canal Zone.

Fort Williams served as the headquarters of the Harbor Defenses of Portland throughout World War II, by the middle of which the last of the coastal artillery pieces (except Battery Keyes' two 3-inch guns) were removed due to age and obsolescence.

[13] In January 1950, with the dissolution of the Coast Artillery Corps, Fort Williams' mission was officially changed from a harbor defense post to a logistical and administrative support installation for all military units and personnel in the State of Maine.

[citation needed] In 1950-51 Fort Williams hosted a station of the Air Defense Command's Lashup Radar Network.

[14] The radar station was deactivated in October 1951 and Fort Williams became an Air National Guard training site.

[1][16][17] On Saturday, 30 June 1962, Fort Williams officially closed and was turned over to the General Services Administration to be sold.