In 1798 he acted as secretary to Sieyès; then after serving for a while in the army, he turned to literature, producing several light plays. Under the Empire he practised with success as an advocate at Bordeaux, where in 1818 he became advocate-general of the cour royale. In 1819 he was appointed procureur-géneral at Limoges, and in 1821 was returned for Marmande to the Chamber of Deputies, where he supported the policy of Villele. In 1822 he was appointed councillor of state, in 1823 he accompanied the duc d'Angouléme to Spain as civil commissary; in 1824 he was created a viscount and appointed director-general of registration.
In contact with practical politics his ultra-royalist views were gradually modified in the direction of the Doctrinaires, and on the fall of Villèle he was selected by Charles X to carry out the new policy of compromise. On the January 4 1828 he was appointed minister of the interior, and, though not bearing the title of president, became the virtual head of the cabinet. He succeeded in passing the act abolishing the press censorship, and in persuading the king to sign the ordinances of June 16 1828 on the Jesuits and the little seminaries.
He was exposed to attack from both the extreme Left and the extreme Right, and when in April 1829 a coalition of these groups defeated him in the chamber, Charles X, who had never believed in the policy he represented, replaced him by the prince de Polignac. In March 1830 Martignac voted with the majority for the address protesting against the famous ordinances; but during the revolution that followed he remained true to his legitimis principles. His last public appearance was in defence of Polignac in the Chamber of Peers in December 1830.
Martignac published Bordeaux au mois de Mars 1815 (Paris, 1830), and an Essai historique sur les revolutions d'Espagne et I'intervention française de 1823 (Paris, 1832). See also Ernest Daudet, Le Ministere de M. de Martignac (Paris, 1875).