St. Leger was active in the Saratoga Campaign, commanding an invasion force that unsuccessfully besieged Fort Stanwix.

St. Leger remained on the frontier for the duration of the war; after its conclusion, he served briefly as commander of British forces in Quebec.

[1][2] He was a younger son of Sir John St Leger, a leading Irish judge and politician, and his second wife Lavina Pennefather.

When word arrived that an American relief column was approaching to break the siege, he sent a group of mostly Indians and Loyalists to oppose them.

They ambushed Nicholas Herkimer's force in the Battle of Oriskany, but the fort's occupants sortied and raided their camp.

He resigned his commission the following year due to ill health, and died in Worksop, Nottinghamshire, England, on 23 December 1793.