He married Françoise Dorothée Argand (1748–1797), with whom he had a daughter, Marie Joséphine Romilly (1770–1823).
In 1770, the Council of State of Geneva granted him his resignation, "retaining him his rank, given his distinguished talents."
Charles Palissot de Montenoy, who knew him specifically, painted a man of gentle and regular manners, highly educated and endowed with the most amiable modesty.
A friend of Rousseau, d'Alembert, Diderot and Voltaire, like his father Jean-Edme Romilly worked for the Encyclopédie to which he contributed the articles Tolérance and Vertu.
He also provided several articles about famous Genevois, among others Jean-Jacques Rousseau, to the Mémoires de littérature by Palissot.