Juin graduated from St. Cyr in 1911 and was posted to Morocco, returned to France for service in World War I and then back to North Africa. Juin was Marshal Lyautey's aide de camp in Morocco, took part in the Rif campaign in 1923 and later headed the Resident General's war council. He took charge of planning the 1929 to ‘33 pacification campaign against the tribes in the Atlas Mountains.
Juin was promoted to General in 1938, taken prisoner in 1940 and released in June 1941 at the request of Marshal Petain who appointed him to succeeded General Weygand as commander of Vichy forces in North Africa. He placed himself under General Giraud's command after the Allied Landings in November 1942.
Juin directed Free French operations against Axis forces in Tunisia in 1943 and formed an expeditionary corps that fought in Italy. He won an important battle at Garigliano and took part in the liberation of Rome along side the Americans and British.