Fernán was the son of Gonzalo Fernández, who had been named count of Arlanza and the Duero around the year 900, and by tradition a descendant of semi-legendary judge Nuño Rasura.

The same year, Álvaro Herraméliz, count of Álava, Cerezo and Lantarón died, and Fernán succeeded him as well, thereby reuniting Rodrigo's county.

Fernán gathered under his control a strong military force composed of troops from the counties of Burgos, Asturias, Santillana, Lantaron, Álava, Castile, and Lara.

Shortly after 4 September 959, his wife Sancha of Navarre died, and in 960 Fernán was defeated through Navarrese intervention on Sancho's behalf.

He was captured by García Sánchez I of Pamplona, but he recovered his freedom after making various territorial concessions and by forswearing his support for Ordoño IV.

After Fernán's death the county was left to his son García Fernández, while his widow Urraca returned to the Pamplona court before marrying William II Sánchez of Gascony.