Sancha of León (born 1191 or 1192) was the eldest child and daughter of Alfonso IX of León by his first queen, Theresa of Portugal. She was raised at the court of their father, but her younger sister Dulce and her brother Ferdinand were raised with their mother in Portugal after the annulment of their parents' marriage in 1195. Although in Castile it was customary for females to inherit, and Sancha's stepmother became queen, briefly, of Castile, in León female succession was barred, although her ancestor Urraca had been the first queen regnant of Western Europe.
After the death of Sancha's brother (1214), Alfonso IX named his second son, also Ferdinand, his heir, bestowing on him the title infante (1216). In 1217, with the support of the aristocracy, Alfonso granted his daughters Sancha and Dulce the villages of Portela de San Juan, Burgo de Ribadavia and Allariz, to be ruled by them until their deaths, after which they would revert to the Crown. In that same year, Ferdinand's mother, Berengaria, inherited the Kingdom of Castile, but ceded it to her son, who was proclaimed king at Valladolid on 2 July. With his heir out of the kingdom and ruling in another place, Alfonso attempted to make his eldest daughters his joint heirs. In the Treaty of Boronal concluded with Portugal in 1219, Alfonso expressly states that if he should die, Portugal should respect the agreement with his daughters.
Alfonso also attempted to secure his eldest daughter's rights by marrying her to John of Brienne, the former King of Jerusalem, but his wife Berengaria blocked this action in order to advance her son. In the spring of 1224, John was on the Way of Saint James, passing through Alfonso's kingdom, when he decided to stop in Toledo in order to see his prospective bride. There the queen convinced him instead to marry her daughter, Berengaria, and on his return trip, at Burgos, he did, his wife and he being accompanied as far as Logroño by the king and former queen of Castile. After this fiasco, Alfonso declared Sancha and Dulce his heirs, but upon his death on 24 September 1230, the people of León, who had pledged for Ferdinand in 1206, refused to recognise his daughters, and they in turn ceded their rights to his kingdom to their half-brother. She was 38 years old at the time. This agreement, negotiated at Valencia de Don Juan by Berengaria and Theresa, with Sancha and Dulce present, is known as the "pact of the mothers". The treaty was signed Benavente and in compensation Ferdinand promised a yearly stipend of 30,000 maravedíes to each of his half-sisters and the lordship of certain castles.