Jean-Bernard Gauthier was born in a family of jurists in Bourg-en-Bresse (now Ain département) of France.
Then, with General George Washington, he served in the Sullivan Expedition against the 'six Indians Nations', up to the current Wyoming.
Under French Brigadier General Marquis de Choisy, he took part to the besieging of York and Gloucester in 1781.
Following a special decision of the US Congress, he was commissioned in September 1783 lieutenant colonel by brevet by the Continental Army.
Promoted as Brigade-General, he commanded the right flank of the Northern Army with General Adam Philippe Custine.
For his duties, he received Croix de Saint-Louis in 1791, and for his services during the American Revolutionary War he became member of the Society of the Cincinnati in 1783.