In 1789 Kellermann enthusiastically embraced the cause of the French Revolution, and in 1791 became general of the army in Alsace. In April 1792 he was made a lieutenant-general, and in August of the same year there came to him the opportunity of his lifetime. He rose to the occasion, and his victory over the Prussians at the Battle of Valmy, in Goethe's words, "opened a new era in the history of the world". Napoleon later commented that: "I think I'm the boldest general that ever lived, but I daren't take post on that ridge with windmill at Valmy (where Kellermann took position) in 1793."
Transferred to the army on the Moselle, Kellermann was accused by General Custine of neglecting to support his operations on the Rhine; but he was acquitted at the bar of the National Convention in Paris, and placed at the head of the army of the Alps and of Italy, in which position he showed himself a careful commander and excellent administrator.
Shortly afterwards he received instructions to reduce Lyon, then in revolt against the Convention, but shortly after the surrender he was imprisoned in Paris for thirteen months. Once more honourably acquitted, he was reinstated in his command, and did good service in maintaining the south-eastern border against the Austrians until his army was merged into that of General Bonaparte in Italy.