[citation needed] Mécia was married on 29 September 1234 to Álvaro Pérez de Castro, a magnate involved in the expansion of the Castilian kingdom into the region of Cordoba, whose first marriage to Aurembiaix, Countess of Urgell had been annulled in 1228,[1] while Mécia's sister Teresa married Nuño, Count of Rousillon, a kinsman of the powerful House of Lara.

These consanguineous marriages were the cause of a dispute between Ferdinand III and Mécia's father Lope Díaz II Haro and husband, the monarch confiscating some of the groom's lands.

[3] The abandonment of agriculture due to the conflict led to a localized famine, and Álvaro was forced to journey to the royal court to plead for assistance.

[3] In response, Mécia sent word to the missing troops, and according to Spanish historian Lafuente she then dressed her ladies in soldiers' arms and paraded them around the battlements.

Having anticipated facing women and not armed men, the Moors slowed their approach and took defensive measures, allowing the missing Christian troops under Tello to return.

They then launched a directed attack under the command of Diego Perez de Vargas which broke through the center of the enemy lines, dispersing them.

It does not appear in the work of Rodrigo of Toledo, completed in March 1243 but also omitting Portuguese royal marriages from the year before, while the papal bull mentions it in 1245, suggesting it should be placed between these dates.

She exacerbated her political isolation by surrounding herself with Castilian-raised servants and maids, making it difficult for Portuguese courtiers to use Mécia as an avenue for approaching the king.

Mécia thus entered the Portuguese stage at the start of a period of political instability, a civil war that eventually resulted in the deposition of her husband, and the coincidence led to her being blamed for his downfall.

Afonso took this to the pope, who issued a bull that decried the state of the kingdom, and backed by complaints from the Portuguese bishops, threatened that unless Sancho lived up to his responsibilities, "appropriate measures" would be taken.

There is a surviving document, dated 24 February 1257, which while not specifying the location, reports Mécia and her relative Rodrigo González acting as executors to Theresa Aires, making certain gifts to the convent of Benavides, suggesting that at the time she resided in the area of Castile.

According to tradition, Mécia died in 1270 at Palencia, where she held lands, and she was buried at Nájera in the Benedictine convent of Santa Maria, in the Chapel of the Cross.