Mary McMartin was born to parents John McMartin and Ellen (Cameron) McMartin in 1778 in Martintown, Glengarry, Ontario in Canada. Between 1799 and 1815 she resided in Charlottenburgh, Glengarry, Ontario.
On December 17, 1799 she was married to Duncan Grant in St. Andrews United Church in Williamstown, Glengarry, Ontario. The church (built in 1787) was built of logs. Its furnishings were primitive; they had planks on cedar blocks for benches.
Duncan and Mary had nine children: John (b. February 20, 1801), Elizabeth (b. July 16, 1803, Angus (Laird) (b. 1805), Nancy (Anne) (b. October 11, 1808), Nellie (Helen) (b. April 4, 1811), Peter (b. 1813), Margaret (b. May 6, 1815), Jennie (b. July 12, 1821), and Duncan (Snowball) (b. April 10, 1823). They built their first and second houses on the rising of their land south of Angus Grant’s Creek.