Kœnig fought in the French Army during World War I and served with distinction. He obtained his baccalaureate and enlisted in 1917. He served in the 36th Infantry Regiment. Marie-Pierre Kœnig was designated as an aspirant in February 1918 and joined his unit at the front. Decorated with the Medaille militarie, he was promoted to lieutenant on September 3, 1918.
Marie-Pierre Kœnig was a captain and assistant to lieutenant-colonel Raoul Magrin-Vernerey in the 13th Demi-Brigade of Foreign Legion of the French Foreign Legion when he decided to engage in Free France on July 1940.
When World War II broke out, Marie-Pierre Kœnig returned to France. In 1940, he was assigned as a captain with the French troops in Norway, for which he was later awarded the Krigskorset med Sverd or Norwegian War Cross with Sword, in 1942. After the fall of France, he escaped to England from Brittany. In London, Marie-Pierre Kœnig joined General Charles de Gaulle and was promoted to colonel. He became chief of staff in the first divisions of the Free French Forces. In 1941, he served in the campaigns in Syria and Lebanon. Marie-Pierre Kœnig was later promoted to general and took command of the First French Brigade in Egypt. His unit of 3700 men held ground against five Axis divisions for 16 days at the Battle of Bir Hakeim until they were ordered to evacuate on 11 June 1942. General de Gaulle said to Kœnig: "Hear and tell your troops: the whole of France is watching you, you are our pride."
Later, Kœnig served as the Free French delegate to the Allied headquarters under General Dwight D. Eisenhower. In 1944, he was given command of the Free French who participated in the Invasion of Normandy. Marie-Pierre Kœnig also served as a military advisor to de Gaulle. In June 1944, he was given command of the French Forces of the Interior to unify various French Resistance groups under de Gaulle's control. Under his command, the FFI stopped range battle in the Maquis, preferring sabotage waged in support of the invasion army. Important in D-Day, the role of the FFI became decisive in the battle for Normandy and in the landing in Provence of the US Seventh Army and French Army B. On 21 August 1944, de Gaulle appointed Marie-Pierre Kœnig military governor of Paris to restore law and order. In 1945, he was sent to arrest Marshal Pétain, who had taken refuge in Germany, but who gave himself up at the frontier with Switzerland.
After the war, Marie-Pierre Kœnig was commander of the French army in the French occupation zone of Germany from 1945 to 1949. In 1949, he became inspector general in North Africa and in 1950 vice-president of the Supreme War Council.
In 1951, after his retirement from the army, Kœnig was elected as Gaullist representative to the French National Assembly and briefly served as Minister of Defense under Pierre Mendès-France (1954) and Edgar Faure (1955).
Marie-Pierre Koenig gave his support to the new State of Israel as president of the Franco-Israeli Committee (Comité franco-israélien). Kœnig had witnessed the heroism of a battalion of Palestinian Jewish mine layers during the Battle of Bir Hakeim and afterwards allowed them to fly their own Star-of-David flag, against British regulations.