He was the fourth child of King Louis VIII and Queen Blanche of Castile. Since the first three children died at an early age, Louis became heir to the throne. He was raised with particular care by his parents, especially his mother, who instructed him deeply in the Christian faith.

When Louis VIII died in 1226, Louis IX was only 11 years old and became king under the regency of his mother, Blanche of Castile. He was crowned king on August 29, 1226, at Reims Cathedral.

In 1234, Blanche relinquished her authority, and Louis IX began his personal rule. He proved to be a just king, known for his kindness, engaging manner, and fairness. He often administered justice personally and was also known for founding many hospitals.

In 1248, Louis IX led the Seventh Crusade to the Holy Land, despite limited enthusiasm among his barons and followers. He sailed from Aigues-Mortes with a fleet of about 100 ships carrying 35,000 men. The crusaders landed near Damietta, Egypt, in June 1249.

However, the crusade was ultimately unsuccessful. Louis IX was captured by Egyptian forces on April 7, 1250. He was released after a large ransom was paid and rejoined his wife at Acre. Louis then spent four years in Syria, turning the military setback into a diplomatic success by forming advantageous alliances and fortifying Christian cities.

He returned to France in 1254 and continued to rule until his death. In 1270, he led another crusade to North Africa. However, the army camp was devastated by disease, and Louis IX himself died on August 25, 1270, at the age of 56.

He was canonized 27 years later, on August 11, 1297.