On the death of her father, Mafalda, under the provisions of his will, was to receive the Seia Castle and the remaining portion of the municipality as well as all income produced there.

This created a conflict with her brother Afonso II O Gordo, who, wanting a centralized power, hindered his sister from receiving the titles and the corresponding rights.

Afonso feared that something similar could happen with his two sisters, Teresa and Sancha, and their eventual heirs, creating a problem of sovereignty that could come to divide the country.

Returning from a pilgrimage to the shrine of Our Lady of Silva, she fell ill at Rio Tinto, Gondomar, and died at the monastery of Cistercians monks there on 1 May 1256.

In 1616, wanting to return her body to Arouca as part of the process of her possible canonization, it was found not to have deteriorated, which generated a strong devotion to the Portuguese princess.