The Chronicon de Cardeña names “Infant D. Sancho” as son of “Rey D. Alfonso”. Alfonso X King of Castile "con la Reyna donna Yolant mi mujer et con nuestro ffijo el Infante don Ferrando primero et yeredero e con nuestro ffijo el Inffante don Sancho" granted rights of the church of Valladolid by charter dated 9 Jul 1258.
Mayordomo of his father King Alfonso X -7 Jul 1277. He fought with his brother Fernando de la Cerda 1275 against the king of Morocco who had invaded southern Spain. After his brother’s death, the nobles recognised Sancho as heir to the throne over the former’s infant children, confirmed by a Cortes at Segovia 1276. Alfonso X King of Castile "con la Reyna doña Yolant mi muger y con nuestros fijos el Infante don Sancho fijo mayor y heredero, y con don Pedro, y D. John, y D. Jaymes" donated "la villa de Caztalla" to the Order of Calatrava by charter dated 1279.
Sancho rebelled against his father together with his brother Juan, and had his father deposed and himself proclaimed King of Castile 1282 by a Cortes at Valladolid. After further disputes with his father, he succeeded in 1284 as SANCHO IV "el Bravo" King of Castile and León.
Betrothed (6 Feb 1270, contract broken before Jun 1281) to Guillelma de Béarn, daughter of Gaston VII de Moncada Vicomte de Béarn & his first wife Mathe [Amata] de Marsan [Mastas] Ctss de Bigorre. The contract between "domini Gastonis…vicecomitis Bearnensis" and "Alfonso…regi Castellæ et dominæ Violant reginæ Castellæ…uxori" provides for the marriage between "Guillelmam filiam" of the former to "infanti domino Sancio filio" of the latter, witnessed by "domino Lop…domino de Biscaia, Didaco Lopi de Haro…", dated 6 Feb 1270.
Mistress (1): María was the wife of Juan Garcia, Señor de Usero, and daughter of Alfonso Alfonso de Meneses and his wife Mayor González Girón. The Nobiliario of Pedro Conde de Barcelos names "D. Maria Alonso llamado de Usero" as the child of "D. Alonso Tellez Tizon", recording that she married "D. Juan Garcia Usero, señor del lugar deste nombre" and was afterwards mistress of "el Rey D. Sancho de Castilla", an earlier passage naming "D. María de Ucero" as the mother of King Sancho´s daughter "D. Violante Sanchez". Her name is confirmed by the charter dated 17 Apr 1293 under which "Fernán Ruiz de Castro pertiguero mayor de Tierra de Santiago" granted arras to "su mujer hija del rey Sancho IV y de doña María Alfón Uceiro".
The Chronicon Domini Joannis Emmanuelis records the death in Apr 1295 of “Rex Dns Sancius Toleti”. The Chronicon Conimbricensi records the death “III Kal Mai” in 1295 of “Dñs Sancius Rex Castellæ”. 1