By the terms of his father's will he received an appanage of Poitou and Auvergne. He won the battle of Taillebourg in the Saintonge War with his brother Louis IX, against a revolt allied with king Henry III of England.
He took part in two crusades with his brother, St Louis, in 1248 (the Seventh Crusade) and in 1270 (the Eighth Crusade). For the first of these, he raised a large sum and a substantial force, arriving in Damietta on October 24, 1249, after the town had already been captured. He sailed for home on August 10, 1250. His father-in-law had died while he was away, and he went directly to Toulouse to take possession. There was some resistance to his accession as count, which was suppressed with the help of his mother Blanche of Castile who was acting as regent in the absence of Louis IX. The county of Toulouse, since them, was joined to the Alphonse's appanage.
In 1252, on the death of his mother, Blanche of Castile, he was joint regent with Charles of Anjou until the return of Louis IX. During that time he took a great part in the campaigns and negotiations which led to the Treaty of Paris in 1259, under which King Henry III of England recognized his loss of continental territory to France (including Normandy, Maine, Anjou, and Poitou) in exchange for France withdrawing support from English rebels.
His main work was on his own estates. There he repaired the evils of the Albigensian war and made a first attempt at administrative centralization, thus preparing the way for union with the crown. The charter known as "Alphonsine," granted to the town of Riom, became the code of public law for Auvergne. Honest and moderate, protecting the middle classes against exactions of the nobles, he exercised a happy influence upon the south, in spite of his naturally despotic character and his continual and pressing need of money. He is noted for ordering the first recorded local expulsion of Jews, when he did so in Poitou in 1249.
Aside from the crusades, Alfonso stayed primarily in Paris, governing his estates by officials, inspectors who reviewed the officials work, and a constant stream of messages.
When Louis IX again engaged in a crusade (the Eighth Crusade), Alphonse again raised a large sum of money and accompanied his brother.. This time, however, he did not return to France, dying while on his way back, probably at Savona in Italy, on August 21, 1271.
His death without heirs raised some questions as to the succession to his lands. One possibility was that they should revert to the crown, another that they should be redistributed to his family. The latter was claimed by Charles of Anjou, but in 1283 Parlement decided that the County of Toulouse should revert to the crown, if there were no male heirs. Alfonso's wife Joan (who died five days after Alfonso) had attempted to dispose of some of her inherited lands in her will. Joan was the only surviving child and heiress of Raymond VII, Count of Toulouse, Duke of Narbonne, and Marquis of Provence, so under Provencal and French law, the lands should have gone to her nearest male relative. But, her will was invalidated by Parlement in 1274 One specific bequest in Alfonso's will, giving his wife's lands in the Comtat Venaissin to the Holy See, was allowed, and it became a Papal territory, a status that it retained until 1791.