He was born in the Thirteen Colonies around 1764; he was living in Vermont at the start of the American Revolution and served with the local militia.
In 1812, with Joseph Willcocks, he resisted efforts by Isaac Brock to pass legislation preparing for war with the United States.
In 1813, Mallory joined the Canadian Volunteers, a militia formed by Willcocks which fought for the American side during the War of 1812.
On 19 December, the British stormed Fort Niagara and launched a raid into New York under Major General Phineas Riall.
In the Battle of Lewiston, Canadian Volunteers under Major Benajah Mallory conducted a fighting retreat for two days, contesting every inch of ground as they fell back to the Tonawanda Creek, where they set fire to the bridge over the unfordable creek to halt the British pursuit.