Leber initially displayed a keen interest in military service. In 1914, he volunteered for the army and served as a soldier during the First World War, where he was wounded and suffered a gas injury. After the war, Leber continued serving in the Reichswehr but was discharged following his opposition to the Kapp Putsch, during which he stood firmly on the side of the Republic.

After his military career ended, Leber resumed his studies, eventually earning a doctorate in political science from the University of Freiburg. He later became the editor-in-chief of a newspaper in Lübeck. As an SPD Reichstag member from 1924 to 1933, Leber focused primarily on military policy.

A staunch opponent of National Socialism, Leber was first arrested in 1933, despite his parliamentary immunity. This marked the beginning of a series of arrests and imprisonments. In 1935, he was placed in protective custody, first at the Esterwegen concentration camp and later at Sachsenhausen. After his release in 1937, Leber joined his wife in Berlin and, in 1940, met Claus von Stauffenberg. He subsequently became an active member of the resistance.

On 5 July 1944, the Gestapo arrested Leber. His trial before the Volksgerichtshof, a sham court, resulted in a death sentence on 20 October. Julius Leber was executed on 5 January 1945 at the Berlin-Plötzensee prison.