The Codex de Roda names "Sanzio rex et domna Gilbira Deuo vota" as the children of "domni Ranimiri regis, frater Adefonsi regis et Froila" and his wife Urraca. The Crónica de Sampiro (interpolated, España Sagrada edition) records that “Ranimirus…Rex” and “Tarasia Regina cognomento Florentina” had “Ordonium, Sanctium et Geloiram”. “Ranimirus…rex” donated property to Celanova by charter dated 11 Aug 941, confirmed by “Hurraca regina, Hordonius prolis regis, Santius prolis regis”. "Ranimirus rex" donated property to Sahagún by charter dated 29 Mar 945, subscribed by "Veremudus rex proles Adefonsi principis, Ordonius proles regis, Sancius frater Ordonius proles regis, Osorio Munniz, Assur Fernandiz, Aurelius Baroncelli". “Ranimirus rex et Urracha regina et prolis eius Ordonius regis” donated "villa...Perales et Volpiare et Villa Rezmiro secundum obtinuit illas Gundisalbo Moniz" to Vega by charter dated 24 Sep 946, confirmed by “...Sanzius filius regis, Gelvira prolis regis et Deo vota...”. "Garsea rex…cum genitrice Tuta regina" donated property to the monastery of San Martín de Albelda by charter dated 22 Nov 947, witnessed by "Santius Ranimiri regis filius…". The Crónica de Sampiro, as reproduced in the Historia Silense, records that "frater…eius…Sancius" challenged the succession of King Ordoño III with "avunculo suo…Garsiano rege Pampilonensium, necnon Fredenandus Gundissalui Burgensium comes" and succeeded in expelling his brother from León. Sancho is generally shown in secondary sources as the son of his father's first wife. However, this passage in the Crónica de Sampiro indicates that Sancho was the son of his father's second marriage, as García III King of Navarre would then have been Sancho's maternal uncle. This relationship would also explain the alliance with Fernando González Conde de Castilla, who was married to the sister of King García III, and also why Sancho later sought refuge in Navarre following his expulsion from León. This speculation is assumed to be correct until a further source is identified which names his father's first wife as Sancho's mother. “Hordonius prolis regis, Sancius prolis regis, Rudericus Uelasquiz, Hordonius prolis domni Adefonsi regis” confirmed the charter dated 13 Jun 950 under which Bishop Rosendo agreed territorial boundaries with the inhabitants of Villaza.
He succeeded his brother in 956 as SANCHO I “el Craso” King of León. The Crónica de Sampiro, as reproduced in the Historia Silense, records that "frater eius Santius, Ramiri filius" peacefully succeeded to the throne on the death of King Ordoño III in "era DCCCCXCIII". Faced with King Sancho's unwillingness to renew his predecessor's peace agreement with the caliph, the latter launched another campaign against León in 957. In 958 Galician nobles dissatisfied with his rule deposed King Sancho in favour of his cousin King Ordoño IV. He sought refuge in Navarre, where he was cured of his obesity by the Jewish doctor Yusuf Hasday ben Sahprut. Allied with García III Sánchez King of Navarre and Caliph Abd al-Rahman III, he took Zamora in 959, forced King Ordoño IV to flee to Asturias and retook the throne in 960. "Sanctius rex" donated property to the monastery of Sahagún by charter dated 26 Apr 960, confirmed by "Tarasia, Gilvira Ranimiri prolis". After his rival ex-King Ordoño IV convinced Caliph Al-Hakam II to provide military support for his restoration in León, King Sancho sent ambassadors to the caliph and persuaded him to withdraw his support in return for accepting the caliph's suzerainty. After ex-King Ordoño died later in 962, King Sancho reneged on his pact with the caliph and made an alliance with the conde de Castilla, the king of Navarre and the conde de Barcelona.
The Crónica de Sampiro, as reproduced in the Historia Silense, records that "Gundissalvus…dux…ultra flumen Dorii" poisoned Sancho I King of León who was trying to exact tribute. The Chronicon Regum Legionensium names "King Ordoño…and King Sancho" as the sons of "King Ramiro", when reporting their reburial in light of the threatened invasion of the kingdom of León and Asturias by Al-Mansur. The Chronicon Compostellani records that “Sancius” reigned 12 years. 1
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