Francois began the war stationed in the province of East Prussia, where he was commander of the I Corps of the German Eighth Army. His task was to defend the easternmost regions of East Prussia against a Russian attack directed at the key city of Königsberg.
When war broke out in August 1914, Francois' corps faced the right wing of a two-pronged Russian invasion of East Prussia, led by Pavel Rennenkampf's Russian First Army. On August 17, the overall German theatre commander, General Maximilian Prittwitz, ordered Francois to retreat while under heavy attack from Rennenkampf.
Francois felt breaking off while engaged would be deadly, and so he ignored Prittwitz' order, responding with the famous reply "General von Francois will withdraw when he has defeated the Russians!" He counterattacked Rennenkampf's massive army, bringing on the Battle of Stalluponen, and won a surprising victory while infliciting 5,000 casualties and taking 3,000 prisoners.
After winning the battle, Francois obeyed Prittwitz' order and withdrew 15 miles to the west, where three days later he fought Rennenkampf to a draw at the Battle of Gumbinnen. Francois' aggressiveness resulted in the cautious Rennenkampf halting his advance westward.
Following that battle and a change of overall commanders, Francois' corps was transferred to the southwest, to confront the Russian Second Army advancing into southern East Prussia under the command of General Alexander Samsonov. Although not trusted by the new German commanders Paul von Hindenburg and Erich von Ludendorff due to his previous disobedience, Francois played the decisive role in the upcoming Battle of Tannenberg (1914).
On August 27, Francois attacked the lead elements of Samsonov's army and began to make steady advances into their rear. Ludendorff, fearing a Russian counterattack by Rennenkampf, now ordered him to break off the advance. However, Francois twice ignored his direct orders and executed an encirclement of Samsonov's army, taking 95,000 prisoners.
Francois' disobedience was forgotten for the time being, and he was given command of the entire Eighth Army for the battles of the Masurian Lakes the following winter. His army played the lead role in winning the Second Battle of the Masurian Lakes on February 7, 1915.
In spite of his success, he was considered unreliable by Ludendorff. Francois returned to corps command for the remainder of the war, but continued to distinguish himself. He won the Pour le Merite, Germany's highest military decoration, in May 1915, and had the Oak Leaves attached to it in July 1917, for outstanding perfomance during the Battle of Verdun.
After the war ended, he returned home and wrote several books on military history, including the best-seller (in Germany) Marneschlacht und Tannenberg in 1920.
While alive, Francois was well known for his French-sounding name, which was seemingly incongruous with his service to Germany. He was descended from a Huguenot family who emigrated to Prussia from France in the 17th century.