His father died early and he was sent to the minor seminary of Aix, where he became friends with Joseph Fesch, the uncle of Napoleon Bonaparte,[3] but did not finish his ecclesiastical studies.

[4] Returning after Napoleon established the Consulate, he accompanied Fesch, meanwhile French ambassador to the Pope, to Rome, and was appointed auditor of the Roman Rota at his instigation in 1803.

The new French king Louis XVIII intended to get Louis-Siffren-Joseph de Salamon to be appointed as an auditor of the Rota, replacing d'Isoard, but the Curia rejected these attempts.

[6] During his time as archbishop of Auch, as a cardinal he participated in the conclaves that elected Pius VIII and Gregory XVI,[8] being charged with possibly pronouncing a French veto in the latter.

[6] Having rejected the archepiscopal seat of Aix twice, and of Bordeaux, he eventually accepted to become archbishop of Lyon in 1839, as successor of Cardinal Fesch, mainly because the climate there was more favourable to his decreasing health.