Born to a farming family in Mercy-le-Haut, he attended the Ecole Polytechnique and the Ecole Superieure des Mines, graduating from both at the top of his class. He then became a mining engineer in Vesoul and Nancy, but left that profession at the age of 29 to enter politics.
Lebrun gained a seat in the Chamber of Deputies in 1900 as a member of the Left Republican Party, later serving on the cabinet as Minister for the Colonies from 1911-1914 and Minister for Liberated Regions, 1917-1919. Joining the Democratic Alliance, he was elected to the French senate in 1920, and served as president of that body from 1931-1932.
Lebrun was elected president of France following the assassination of president Paul Doumer by Pavel Gurgulov on May 6, 1932. Re-elected in 1939, largely because of his record of accommodating all political sides, he exercised little power as president and was forced to accept the German terms of surrender in on July 10, 1940, after which he was replaced by Henri-Philippe Pétain (although Lebrun never officially resigned).
He then fled to Vizille (Isère) on July 15, but was captured on August 27, 1943 when the Germans moved into the region and was sent into captivity at the Itter Castle in the Tyrol. On October 10, 1943 he was allowed to return to Vizelle due to poor health, but was kept under constant surveillance.
On August 9, 1944, when the Allies restored the French government, Lebrun met with Charles de Gaulle and acknowledged the General's leadership, saying that he had not formally resigned as president because the dissolution of the National Assembly had left nobody to accept his resignation.
After the war, Lebrun lived in retirement. He died of pneumonia in Paris after a protracted illness.