Martigues began his political career serving under his uncle Étampes the governor of Brittany as the lieutenant-general of the province.

He was further allowed to inherit the estates of his uncle that had been confiscated during the League of the Public Weal, making him first comte then duc de Penthièvre.

Martigues played an important role on the right of the royal army, commanding a block of veteran French infantry.

[7] He and his troops were held in reserve for much of the battle, until it was deployed late in the day, when the Huguenot cavalry looked like it might overwhelm the crowns forces.

Martigues' troops formed up a square and held the cavalry off, after which Coligny ordered the Protestant forces to retreat from the field.

[12] As the peace broke down, Montgommery and the Vidame de Chartres moved south from Brittany pursued by Martigues.

[13] With the resumption of civil war in late 1568, the Protestant nobility fled south to regroup at La Rochelle, to provide support to them an army assembled in Provence and began marching north.