During the Seven Years' War he commanded that regiment in North America, where it fought at the capture of Louisbourg and the Battle of the Plains of Abraham.
He was promoted to the rank of colonel in 1760, and appointed to the colonelcy of the 56th Regiment of Foot in 1766, a post he would hold for almost thirty years.
As a reward for his military service, Lot 11, Prince Edward Island was awarded to Colonel Walsh in the great lottery of 1767.
While ownership remained with the heirs of Colonel Walsh, portions of the lot were leased to settlers under the administration of land agents in Prince Edward Island.
In 1856, the Walsh heirs sold the lot to the Prince Edward Island colonial government for resale to leaseholders in accordance with the Land Purchase Act of 1853.