Louis Caesar Constantine de Rohan-Guemène belonged to the House of Rohan, an ancient and powerful family of the nobility of Brittany which dates back to the xi th century.

His brother Armand-Jules de Rohan-Guémené, future archbishop of Reims, had chosen the ecclesiastical state.

On the death in 1756 of his uncle, Armand de Rohan-Soubise, he was appointed to succeed him as Bishop of Strasbourg, a post which carried with it the title of Prince of the Holy Roman Empire (though most of the principality's territory situated on the left bank of the Rhine had been annexed by France in the late 17th century) and likewise the title of Count of Alsace.

However, he was not made Grand Almoner of France, unlike other members of the Rohan family who became bishops of Strasbourg.

His cousin and successor as Cardinal Prince-Bishop of Strasbourg, Louis-René-Édouard de Rohan-Guémené, would be compromised in the Affair of the Diamond Necklace.