Mendo succeeded his father, who died in 1028, in the governance of the County of Portugal, most probably as a minor under the tutorship of his mother Ilduara. Less than a decade later, in 1037 Ferdinand I acceded to the throne of León after defeating and killing his brother-in-law King Vermudo III in the Battle of Tamarón. By 1050, the new king had reorganized the administration of the kingdom curtailing the power of the nobility and royal appointees. He achieved this mainly by converting counties into non-hereditary tenures and taking advantage of any opportunity that arose to appoint new governors. The king did this gradually, region by region, so as not to alienate the powerful nobility of the kingdom and, in the case of the County of Portugal, by trying not to antagonize the powerful Galician and Portuguese members of the high nobility who were related to Count Mendo Nunes who was also a first cousin of his wife, QueenSancha of León. During count Mendo's lifetime, the king started to appoint members of the lower nobility to administrative posts such as Gómez Ectaz, who exercised authority in the region of Guimarães, and Diego Tructesíndez who acted as a judge, both reporting directly to the monarch. This practice became more widespread after Mendo's death with the king naming members of the lower ranks of the nobility to administrative positions, with various titles, such as vicar or governor, for example, Godino Benegas attested in 1062 as governor of Portugal: Gutinus Veniegas, qui tenebat illa terra de Portugale de ille rex. Years later, Mendo's granddaughter, Loba, married Sisnando Davides, a Mozarab of unknown background and certainly not of noble lineage, who was appointed by King Ferdinand governor of the county of Coimbra and who never entitled himself count preferring to use the titles of alvasil or consul. The date of Count Mendo's death is uncertain due to a confusion with another contemporary count, Mendo Luz, as well as the erroneous date recorded in the Annales Portugalenses veteres which state that Era MLXXII occisus fuit comes Menendus in ripa Guetanie, i.e. Menendus was killed in Era MLXXII in the Guetania River. Portuguese historian José Mattoso considers that the year of his death, probably violent, was 1050 and no later than 1053 whereas Spanish historian Alfonso Sánchez Candeira, believes that he died on 24 December 1054.
Marriage and issue
The name of his wife is not recorded in any charters. He was succeeded by his son:
