Jacques Courtois was born in Saint-Hippolyte, near Besançon (Franche-Comté) in present-day France, but at the time, a Spanish possession in Holy Roman Empire. He was the son of the obscure painter Jean-Pierre Courtois. Very little is known about Guillaume’s youth but it is assumed he received his initial training from his father. He had two younger brothers who also became painters Guillaume (Guglielmo Cortese) (1628 - 1679) and Jean-François (c. 1627-?). As his brother was later also known as 'il Borgognone' (a reference to their origins in Burgundy, called Comté de Bourgogne or Franche-Comté in French), some of the works of the brothers have been confused.