In 1857 Magniac married Augusta Dawson (?–24 February 1903, née, FitzPatrick), daughter of Baron Castletown, and widow of Lieutenant-Colonel Thomas Vesey Dawson, who died in action at the Battle of Inkerman in 1854. Their daughter, Helen Augusta, married the army officer and explorer Francis Younghusband, to whom her brother, Vernon, served as private secretary on the British expedition to Tibet in 1904.