He came to North America with General Simon Fraser's 71st Regiment which fought for the British in the American Revolution; MacDonell reached the rank of lieutenant.
He appears to have been taken prisoner at Yorktown, Virginia and returned to Britain.
In 1786, he returned to Canada and received a grant of land in Charlottenburgh Township.
He represented Glengarry & Prescott in the 3rd Parliament of Upper Canada.
Later, he is believed to have become a fur trader and left the area.