The Codex de Roda names "Sanzio Garseanis et Scemeno Garseanis" as the children of "Garsea Scemenonis" and his second wife "domna Dadildi de Paliares soror Regimundi comitis", one manuscript stating that Sancho was "cognomento Avarca" (confusing Sancho with his grandson of the same name), succeeded as king in 905, reigned eighteen years and died in 924.
He succeeded in 905 as SANCHO I King of Navarre, after deposing his distant cousin King Fortún García. The Libro de Regla of Leire Monastery, compiled in 1076, records that "frater eius Sancius Garseanes cum uxore sua Dna Tota regina" ruled in place of "Fortunius Garseanes", who retired to Leire. "Sancius rex cum Tuta regina" donated property to the monastery of Leire by charter dated 21 Oct . He ended Navarre's long standing alliance with the Banu Qasi family: in 915, he defeated Muhammed ben Lub at Tudela, obliged him to cede the castles of Falces and Caparroso and give two of his sons as hostages. Allied with Ordoño II King of León, he conquered the Nájera district to the south west around Logroño between 918 and 923, and recaptured Viguera in 923. "Sancius rex, filius Garsie regis, successor in regno germani mei Fortunii…cum domina Tuta regina uxore mea" donated property to the monastery of Leire by charter dated 18 Mar . The Cronica de Sampiro records that "rex…Sancius Garsiani filius" allied himself with "regem domnum Ordonium", although this is not dated. “Rege Santio Garseanis cum suos germanos Enecho Garseanis et Scemeno Garseanis” confirmed the territory of the monastery of Fontfride by charter dated 1 Oct 921, witnessed by “Galindo Galindonis...Sancio Scemenones cum suos germanos et cum suos filios, Blasco Lupi et Blasco Sanciones, Kardellus Ennecones cum filiis suis, Isinarius Gintalli cum filios suos...et scola de rege et de suos germanos, Santio Galindonis...”. Abd al-Rahman III Emir of Córdoba counter-attacked, recaptured Tortosa, Sangüesa and in 924 sacked Pamplona. "Sancius rex…cum coniux mea Tuta regina" donated property to the monastery of San Martín de Albelda by charter dated 5 Jan 925, confirmed by "…Garsea eiusdem principis filius, Enneca eiusdem principis filia, Belaschita eiusdem principis filia, Eneco Garseanis, Semeno Garseanis…Eneco Sanzoniz, Garsea Eneconis".
The Codex de Roda records the death "925 III Id Dec" of "Sanzio Garseanis" after reigning for twenty years. Another part of the Codex de Roda, "Initium regnum Pampilonam", records that "rex…Sancio Garseanis" was buried "in Sancti Stefani portico". The Libro de Regla of Leire Monastery, compiled in 1076, records that "Sancius Garseanes" died "era DCCCCLXVIII". 1