Ecole Spéciale Militaire de Saint-Cyr
Garnier-Duplessix was born on 25 December 1860 at Rennes in Brittany. His parents were Louis Marie Francois and Marguerite Blanche Lucie Boissonnet Garnier-Duplessix. He joined the French Army as a volunteer (initially for a 5-year period only) on 25 October 1880 at Rennes and entered the Ecole Spéciale Militaire de Saint-Cyr on 29 October. Garnier-Duplessix was steadily promoted through the cadet ranks, becoming a first class cadet on 13 April 1881, a corporal on 24 August, Quartermaster-Sergeant on 3 November and Sergeant-Major on 7 December. Upon passing out from the Ecole he joined the 19th Battalion of Light Infantry as a sub-lieutenant on 18 October 1882. He was promoted to lieutenant and transferred to the 47th Battalion on 31 May 1886. Garnier-Duplessix then joined the 7th Infantry Regiment on 23 February 1891.