When the American Revolutionary War broke out, Macarmick at his own expense recruited and established the 93rd Regiment of Foot, winning appointment as its colonel in February 1780.
In 1784 he won election to Parliament representing Truro, a post he held until he was appointed lieutenant governor of Cape Breton Island in 1787.
When the British took control of Cape Breton after the Seven Years' War, attention was focused on the working of the mines on a commercial basis, which were thought to provide an inexhaustible source of revenue.
In its first few years the colonial leadership was beset by political squabbles under Lieutenant Governor J. F. W. DesBarres, and Macarmick was instructed to end the factionalism.
Macarmick requested leave to return to England in 1794, after financial perquisites from the province's extensive coal fields were withdrawn.