Born in 1768 to a father who was a minor government official in Saarlouis (then a French territory), Grenier joined the French royal army as a private soldier in the "Nassau" Infantry Regiment on 21 December 1784. After the French Revolution broke out he continued in the army. He fought at the Battle of Valmy, becoming a captain. After notable actions at the Battle of Jemappes, he received promotion to adjutant general. He was elevated to the rank of general of brigade on 29 April 1794. Grenier later fought at the Battle of Fleurus in Jean Étienne Championnet's division. His brigade included the 18th Line Infantry Regiment and the 1st Dragoon Regiment. For his distinguished actions, he was promoted to general of division on 11 October. He commanded the advance guard at the passage of the Rhine River on 6 September 1795. He led a division at the Battle of Würzburg on 3 September 1796. Under his leadership were three battalions each of the 20th Light, 16th Line, and 67th Line Infantry Demi-Brigades, one battalion of the 23rd Line Infantry Regiment, the 1st and 2nd Dragoon Regiments, the 6th Chasseurs à Cheval Regiment, one foot artillery company, and one horse artillery company. He won praise for his leadership in the Battle of Neuwied on 18 April 1797.