In April 1777, St. Clair was sent to defend Fort Ticonderoga. His outnumbered garrison could not resist British General John Burgoyne's larger force in the Saratoga campaign. St. Clair was forced to retreat at the Siege of Fort Ticonderoga on July 5, 1777. He withdrew his forces and played no further part in the campaign. In 1778 he was court-martialed for the loss of Ticonderoga. The court exonerated him and he returned to duty, although he was no longer given any battlefield commands. He still saw action, however, as an aide-de-camp to General Washington, who retained a high opinion of him. St. Clair was at Yorktown when Lord Cornwallis surrendered his army. During his military service, St. Clair was elected a member of the American Philosophical Society in 1780.