During World War II, the schooner was acquired by the U.S. Coast Guard and used to patrol the New England coast for enemy submarines. During this time, two machine guns were mounted on Brilliant's deck.
After the war, Brilliant was purchased by the accomplished sailor Briggs Cunningham, who attempted to increase her speed by outfitting her with a larger rig. The new rig consisted of taller masts, a self-tacking forestaysail, and a Bermuda-mainsail, replacing the original gaff main. During this time, Cunningham also invented what became known as the cunningham, a tie-down for the tack (the lower, forward corner of the mainsail) that allows the sail to maintain a more efficient shape. This makes Brilliant the first boat to have the device, now standard on racing boats of all sizes. Despite his modifications, Cunningham was unable to significantly improve the Brilliant's speed and he donated her to Mystic Seaport in 1957.
Mystic Seaport now uses her as an offshore classroom and features her as part of their collection of watercraft. This use was a requirement of Cunningham's gift to Mystic Seaport.