He was the eldest son of Nuño Velázquez and Fronilde Sánchez, daughter of Count Sancho Ordóñez.
On 13 June 1104, he appears with his parents and siblings Menendo, Sancho, and Elvira in a donation to the monastery of Sahagún.
Between 1 April 1101 and 24 October 1102 Alfonso served as the alférez of Raymond of Galicia.
He was one of the few Galician magnates loyal to Queen Urraca during her conflict with her son by Raymond, Alfonso Raimúndez, which may be surprising in light of Alfonso Núñez's known political connexions with Raymond.
His importance to Urraca in Galicia is exemplified by the Historia compostellana, which lists him first after Urraca's son when naming the leaders of the army which she assembled among "all the consuls and princes of Galicia"[1] for making war on Alfonso I of Aragon and Navarre in the spring of 1118.