Born in Scotland, Macdonell came to New York in 1773. Commissioned ensign in the Royal Highland Emigrants (84th Regiment) in June 1775 and later transferred to Butler's Rangers, Macdonell served with distinction until 1784 when he settled with his clansmen in Charlottenburg Township. He represented Glengarry in the Legislative Assembly (1792-1800) and was chosen first speaker (1792-96). Macdonell served as Lieutenant of Glengarry (1793-1808), commander of the 2nd Battalion Royal Canadian Volunteers (1796-1802), and paymaster of the 10th Royal Veteran Battalion from 1807 until his death.