Pierre Messemer was born into a family of industrialists, on March 20, 1916, in Vincennes. He studied at Charlemagne College as well as the Grand Louis, receving his bachelor's degree in 1933. He then followed onto to study at the National School of France Overseas, from 1834-37, and received his doctorial degree in 1937, he also attened the School of Oriental Languages. He worked briefly as a colonial administrator, before attending the Saint-Cyr Military Academy in early late 1937. He completed his training in 1939 and was posted with the 12th Reiment of Senegalese Riflemen. Upon the declaration of war, he remained with the 12th Senegalese as a second lieutenant. He was undergoing training at Aulant as an aerial observer, when Petain issued his declaration of intent to surrender in June of 1940. Messmer decides along with another lieutenant, Jean Simon, to continue fighting, and refuse surrender.

They head to the south of France, and by hitch-hiking manage to arrive in Marseilles. They receive sympathy from the commander of the merchant navy in the region, Vuillemin, and are arranged to embark under false identity aboard the Italian cargo liner, Capo Olmo, which is leaving for North Africa. They collect a number of like minded French army officers to join them. After the ship embarks, Messmer and Simon manage to cohersively convince the captain of the Capo Olmo to Gibraltar, after having convinced the crew of the dangers of sailing in the mediterranean, and the inability of the axis to protect their merchant ships en route to Africa. The two eventually arrive in Liverpool on July 17, 1940, along with 30 other recruits they brought with them for the Free French cause. Messmer and Simon also convinced the captain of the Capo Olmo to hand over his cargo, bringing a large stock of raw material, including aircraft parts, which were used to fund the Free French forces. Messmer was posted to the 13th DBLE as a section head of the 3rd Company, under Jacques de Lamaze, and serves with the unit through the campaigns in the later year.

He was noted for valor at Eritrea, where his section seized the slopes of the South of Grand Willy, on the night of March 13-14, 1941. On April 8, at Massaoua, he removed a machine gun position by slowly crawling towards it over open ground, and throwing a grenade. He further led his section to storm two larger enemy positions, capturing 3 officers and 70 marines. For his actions he was decorated with the Croix de Liberation by De Gaulle at camp Qastina in Palestine. During the battle in Syria, he is promoted to captain and commander of the 3rd company. At Bir Hakeim he is later transferred to command one of the companies of the newly formed third battalion of the 13th DBLE. He is commended for maintaining his position against fierce Axis assaults. During the Second Battle for El Alamein, he is credited with firmly defending the position of Nag-rala, inflicting heavy losses on the enemy. Thereafter he follows the unit through Tunisia.