In 1217 Ferdinand became King of Castile. In 1230 he succeeded to the crown of Leon. In 1219 Ferdinand married Beatrice, the daughter of Philip of Swabia, King of Germany. Ferdinand and Beatrice had six sons and one daughter.

The highest aims of Ferdinand's life were the propagation of the Faith and the liberation of Spain. In the most important towns he founded bishoprics, reestablished Catholic worship, built churches, founded monasteries and endowed hospitals. The greatest joys of his life were the conquests of Cordova in 1236 and Seville in 1248. He turned the great mosques of these places into cathedrals and dedicated them to the Blessed Virgin. He founded the University of Salamanca.

Ferdinand was buried in the great cathedral of Seville before the image of the Blessed Virgin clothed in the habit of the Third Order of St. Francis. His body, it is said, remains incorrupt. Many miracles took place at his tomb. Clement X canonized him in 1671.

His feastday is May 30th.