"Garcia Fredinandiz, Aua comitisa" donated property to the monastery of Cardeñas by charter dated 11 Jul 972, subscribed by "Sancius prolis illorum, Domna Fronilde…". "Garssia Ferdinandi…comes et imperator Castelle…cum uxore mea Abba comitisse" granted fuero to Castrojeriz by charter dated 8 Mar 974, subscribed by "Sanctio filio nostra, Urraca filia nostra…". "Garsias Ferdinandez…cum coniuge mea Ava comitisa" founded the monastery of Infantado de Covarrubias, offering "filiam…nostram Urracam" as a nun there, by charter dated 25 Dec 978, confirmed by "Sancio Garsea, Gundisalbo Garsea…".
He rebelled against his father in 990, supported by Ibn Abu Amir, effectively dividing Castile into two parts. The Annales Complutense record that “Sancius Garsia” rebelled against “patrem suum comitem Garsia Fernandez” in 990.
He succeeded in 995 as Conde de Castilla. Ibn-Khaldun states that Alfonso V King of León succeeded under the regency of "le comte de Galice Menendo Gonzalez" but that this was disputed by "Sancho fils de Garcia, l´oncle maternel d´Alphonse", the dispute being arbitrated by "Abdalmelic fils d´Almanzor" who ordered "au juge des chrétiens [de Cordoue] Açbagh ibn --- de décider…[et il] prononça en faveur de Menendo Gonzalez". "Sancio comite et Urraca cometissa soror mea" granted rights to the monastery of San Millán de la Cogolla by charter dated 1003, signed by "…Fernando Beilaz, Oveco Alvarez, Beila Alvaroz, senior Assur Nunniz dominate Ponticurbo, Alvaro Gomiz, Tellu Didaz, Beila Gutier". He allied himself with the Berbers in 1009 to fight and depose Caliph Muhammad. The Chronicon Burgense records that “comes Sancius” destroyed Córdoba in 1009. He was given a formal reception in Córdoba by the new Caliph Sulayman, and received strategic castles as his reward.
Ibn-Khaldun records that "le seigneur d´Alava Sancho, fils de Garcia" was killed "par le roi des Basques" in [21 Jun 1015/9 Jun 1016. The Chronicon Burgense records the death in 1017 of “comes Sancius”. 1