He joined the staff of the Cours des Aides in 1730, during the presidency of his uncle, the abbé Louis Lefranc, who had succeeded his brother, Jean-Jacques' father Jacques, on the death of the latter in 1719. When Louis died in 1745, Jean-Jacques, who had by then served for fifteen years as a general advocate at the court, although expected to succeed him in turn, was not yet old enough to be awarded the position, and had to wait until early 1747 to take over its presidency.