Stewart, his older brother Henry, and younger brother Ludovic were brought up at Aubigny in France as Roman Catholics under the charge of their paternal grandmother, the old Duchess-Dowager, Katherine de Balsac. In 1638 he secretly married Katherine Howard, the daughter of Theophilus Howard, 2nd Earl of Suffolk, and Elizabeth Home, without her father's consent, offending his guardian the king. George Stewart and Katherine Howard had two children: He fought with the French against the Spanish in the Battle of Montjuïc (1641).