His parents were García Sánchez II the Tremulous and Jimena Fernández, daughter of Fernando Bermúdez, count of Cea on the Leonese frontier.

The first record of the future king is a diploma of his father's granting the village of Terrero to the monastery of San Millán de la Cogolla.

This gap has led to speculation as to whether there was an interregnum, while one document shows Sancho Ramírez of Viguera reigning in Pamplona in 1002, perhaps ruling as had Jimeno Garcés during the youth of García Sánchez I three generations earlier.

In 1018, William Isarn tried to solidify his control over the Arán valley, but was killed, and Sancho jumped on the opportunity to take his portion, presumably based on some loose claim derived from his wife.

He had been betrothed to Sancha, daughter of Alfonso V, who was set thus to gain from Castile lands between the rivers Cea and Pisuerga (as the price for approving the marital pact).

Using the pretext of the protectorship he had exercised over Castile, he immediately occupied the county and named as successor his own younger son Ferdinand, who was nephew of the deceased count, bringing it fully within his sphere of influence.

Sancho was soon engaged in a full-scale war with León, and combined Castilian and Navarrese armies quickly overran much of Bermudo's kingdom, occupying Astorga.

However, he died on 18 October 1035 and was buried in the monastery of San Salvador de Oña, an enclave in Burgos, under the inscription Sancius, gratia Dei, Hispaniarum rex.

Ramiro, the eldest but illegitimate son of Sancho by mistress Sancha of Aybar, was given property in the former county of Aragón with the provision that he should ask for no more lands of his brother García, under whom he first acted as baiulus but from whom he later achieved de facto independence.

Taking residence in Nájera instead of the traditional capital of Pamplona, as his realm grew larger, he considered himself a European monarch, establishing relations on the other side of the Pyrenees.

The Mozarabic rite continued to be practiced at San Juan, and the view that Sancho spread the Cluniac usage to other houses in his kingdom has been discredited by Justo Pérez de Urbel.

The division of his realm upon his death, the concepts of vassalage and suzerainty, and the use of the phrase "by the grace of God" (Dei gratia) after his title were imported from France, with which he tried to maintain relations.

At least nominally, he ruled over León, the ancient capital of the kingdom won from the Moors in the eighth century, and Barcelona, the greatest of the Catalan cities.

It is for this that his seal has been appropriated by Basque nationalism, though, by dividing the realm, he isolated the kingdom and inhibited its ability to gain land at the expense of the Muslims.

After his coronation in León, he styled himself rex Dei gratia Hispaniarum, or "by the grace of God, king of the Spains", and may have minted coins with the legend "NAIARA/IMPERATOR".

[a] The use of the first title implied his kingship over all the independently founded Iberian kingdoms and the use of the form Dei gratia, adopted from French practice, stressed that his right to rule was of divine origin and sustenance.