Sherbrooke arrived at Halifax on 16 October 1812 as commander of the forces in the Atlantic provinces. The five years of Sherbrooke's administration were dominated by war with the United States, which broke out in June 1812, and matters relating to the colony's defence. He mounted guns at harbour entrances across the colony and placed the militias in a state of readiness. The war proved to be profitable for Nova Scotia. Sherbrooke's commercial policies turned the Atlantic provinces into a thriving entrepôt for international trade. From 1813–1830 Sherbrooke was Colonel of the 33rd Regiment of Foot.
New Ireland
Deciding to strike at the long-disputed borderland between Passamaquoddy Bay and the Penobscot River, Sherbrooke led an expeditionary force that August which successfully landed at Castine and proceeded to subdue the entire region between the Penobscot and the St Croix. He renamed the region the colony of New Ireland. The eight-month occupation of Castine yielded customs revenues which were subsequently used to finance the Cambridge Military Library in Halifax and found Dalhousie College. The financing of Dalhousie college, now Dalhousie University in Halifax had largely come fromcustom duties collected by Sir John Coape Sherbrooke, then lieutenant-governor of Nova Scotia during the occupation of Castine, Maine during the War of 1812, investing £7000 as the initial endowment and £3000 reserved for the actual construction of the college.
His active defence of the colony during the War of 1812 led to his appointment as Governor General of British North America in 1816. His talent as a mediator helped settle disputes, and he won the confidence of Louis-Joseph Papineau. Ill health forced Sherbrooke to resign after only two years, and he retired to Nottinghamshire in England. However, his brief tenure was remembered as a period of calm before the coming storm. While he resided in Nova Scotia, his home was at Birch Cove on Bedford Basin, near Halifax. Named Sherwood, it eventually lent its name to the neighbourhoods of Sherwood Park and Sherwood Heights. The community of Sherbrooke, Nova Scotia also bears his name. Other honorific eponyms are listed below.
Honorific eponyms
;Geographic locations
Nova Scotia: Sherbrooke
Nova Scotia: New Ross, formerly known as Sherbrooke