At the start of World War I, he was chief of Minister of War Adolphe Messimy's military cabinet.
When the latter left office, Guillaumat was appointed as the head of the 33rd Infantry Division on August 30, 1914, and then of the 4th Infantry Division on December 9, 1914.
He led the 1st Army Corps from February 25, 1915, and on December 15, 1916, he replaced Robert Nivelle as commander of the Second Army, when the latter was made commander-in-chief of the French Army.

Guillaumat was sent to replace General Sarrail as commander of the Allied Army of the Orient in Salonika in December, 1917.
He laid the plans later executed by his replacement, General Franchet d’Esperey, and rebuilt the relations with France's allies somewhat damaged by his predecessor.
Guillaumat was recalled to Paris on 17 June 1918 and replaced with Franchet d’Esperey.

There, he replaced Auguste Dubail as Military Governor of Paris.
After the success of the Second Battle of the Marne, he was appointed to the Supreme War Council.
He then returned to active command as commander of the Fifth Army, which he led through the Ardennes at the end of the war.
At the start of World War I, he was chief of Minister of War Adolphe Messimy's military cabinet. When the latter left office, Guillaumat was appointed as the head of the 33rd Infantry Division on August 30, 1914, and then of the 4th Infantry Division on December 9, 1914. He led the 1st Army Corps from February 25, 1915, and on December 15, 1916, he replaced Robert Nivelle as commander of the Second Army, when the latter was made commander-in-chief of the French Army.
Guillaumat was sent to replace General Sarrail as commander of the Allied Army of the Orient in Salonika in December, 1917. He laid the plans later executed by his replacement, General Franchet d’Esperey, and rebuilt the relations with France's allies somewhat damaged by his predecessor. Guillaumat was recalled to Paris on 17 June 1918 and replaced with Franchet d’Esperey.
There, he replaced Auguste Dubail as Military Governor of Paris. After the success of the Second Battle of the Marne, he was appointed to the Supreme War Council. He then returned to active command as commander of the Fifth Army, which he led through the Ardennes at the end of the war.
Adolphe Guillaumat