Lebrun, a mining engineer, was educated at the Nancy Lycée, the École Polytechnique, and the École Nationale Supérieure des Mines. He was elected deputy for Lorraine in 1900, senator in 1920, and president of the Senate in 1931. Other posts he held during that period included: minister of colonies (1911–13; 1913–14), of war (1913), and of blockade and of liberated regions (1917–19).

Lebrun, himself a moderate conservative, was elected president of the republic on May 10, 1932, largely as a compromise candidate acceptable to all factions. In his role as mediator and as a symbol of unity, Lebrun easily adapted to governments of both the right and the left, rarely exerting political influence on cabinet appointments or policy. On April 15, 1939, Lebrun was reelected president, only the second among the presidents of the Third Republic to be so honoured.

When Germany successfully invaded France early in World War II, Lebrun complied with the cabinet's decisions of June 1940 that led to the armistice with Germany, although he personally would have preferred heading a government-in-exile. In July, Lebrun acquiesced in the constitutional revisions at Vichy through which Marshal Philippe Pétain took over as head of state. Lebrun retired to Vizille near Grenoble and was later interned by the Germans at Itter in Tirol (1943–44). By acknowledging General Charles de Gaulle (Gaulle, Charles de) as head of the provisional government as the Allies liberated France, Lebrun ended his own political career. In his autobiography, Témoignage (1945; “Testimony”), he attempted to clarify the confusing events in which he had participated.