He began his career aged seven aboard the Formidable, and was wounded at the Battle of Quiberon Bay, on 20 November 1759.

After war ended, he sailed again as a merchant, and joined the Navy again in 1787 after a reform of the status of officers from the ranks and files, as a sous-lieutenant de vaisseau.

A resolute Republican, Nielly had his crew sign a manifesto supporting the Constitution of the French First Republic, and sent it to the National Convention.

Nielly was put in charge of the third squadron of the Brest fleet, and took part in the Croisière du Grand Hiver, under Villaret-Joyeuse, and in the Expédition d'Irlande, under Morard de Galles, with his mark on the frigate Résolue [3].

Nielly later held the office positions of commandant d’armes in Brest and Lorient, and Préfet maritime in Dunkirk from 1800.