Mécia Lopes de Haro was born circa 1215 as the daughter of Lope Díaz II de Haro, Lord of Biscay and Urraca Alfonso of Léon, who was an illegitimate daughter of Alfonso IX of Léon. She was first married on 29 September 1234 to Álvaro Pérez de Castro, but she was a childless widow by 1240.
Her second marriage was a lot more surprising. She married King Sancho II of Portugal around 1245. She is considered his mistress by some, but their marriage is documented by a papal bull. The exact date is uncertain. The marriage was not popular. She was not a royal princess, and she was not the ideal virgin bride. Furthermore, she isolated herself with Castilian servants. She became Queen when Portugal was politically unstable, and she is often blamed for Sancho’s subsequent downfall. To embarrass the King, even more, the clergy rose up against his marriage. However, Sancho refused to repudiate his wife and the Pope then annulled the marriage for being within the forbidden degrees of consanguinity.
Mécia was then kidnapped from the royal bedchamber and taken to the Palace at Vila Nova de Ourém. Sancho was powerless to help his wife. He abdicated and fled to Toledo, where he died just three years later. Mécia continued to live at Ourém as there are records of her making donations there.
Mécia died around 1270, and she was buried in the Benedictine convent of Santa Maria at Nájera. Her tomb still survives to this day.