MacDowall hailed from Garthland Mains, Dumfries and Galloway, Scotland, where the family seat was Garthland Castle.[2] He was the fourth son of William MacDowell (c.1719–84), M.P. for Renfrewshire, and Elizabeth Graham, granddaughter of Alexander Livingstone, 3rd Earl of Callendar. His brothers William MacDowall (c.1749–1810) and Captain David McDowall-Grant (1761–1841) were Members of Parliament. His nephew was Lt. Gen.
Day Hort MacDowall (1795–1870) and great-nephew was Canadian politician Day Hort MacDowall (1850–1927).[3]
In August 1782, he was the commanding officer of the fort of Trincomalee when the French lay siege to it in the run-up to the Battle of Trincomalee. He surrendered to Suffren on 30 August in exchange for safe passage to Madras for his 1,000-man garrison.[4][5]