The Chronicle of Alfonso III states that "his son Alfonso" succeeded King Ordoño "in the era 904 (866)". His birth date is estimated from the Chronicon Albeldense which records that he was 17 years old in the first year of his reign. "Adefonsus rex" confirmed the donation to Santiago de Compostela made by "domini Ordoniii…principis" by charter dated 862.
He succeeded his father in 866 as ALFONSO III "el Magno" King of Asturias. His succession was challenged by Fruela Vermúdez, conde in Galicia: the Crónica de Sampiro, as reproduced in the Historia Silense, records the succession of "Adefonsus filius Ordonii" aged 14, the challenge by “filius...Froyla Lemundi ex partibus Gallecie”, King Alfonso´s exile “in partibus Alauensium”, and the death of “ipse...Froyla” killed “a senatu Outensi”. King Alfonso was obliged to seek refuge in Castile with conde Rodrigo until the rebel was killed by the king's supporters: the Chronicon Albeldense records the rebellion of “Froilane, Galliciæ comite” in the first year of Alfonso´s reign, the king´s exile in Castile, and his return after Fruela was killed. He was also faced with a Basque rebellion in 867. Having pacified internal opposition, he was able to turn his attention to territorial expansion and reconquest from the Moors. He occupied northern Portugal in 868, and resettled the land between the Miño and Duero rivers. “Adefonsus rex” donated “ecclesiam...Sanctæ Mariæ...in villa...Tenciana” to “Sisnando presbytero” by charter dated 15 Apr 869, confirmed by “...Hermegildus, Sigericus”. A charter dated 6 Jun 878 records judgment in the court of “domini...Adephonsi principis...vel judicum Gatoni et Hermigildi” confirmed the possession of “villa Vimineta”, populated by “comite Gaton” during the reign of “domni Ordonii”. By 880, all of Galicia and the northern third of Portugal were under his control. Despite slower progress in Castile and León, King Alfonso was able to force territorial concessions under the peace of 884. Taking advantage of internal conflicts among the Muslims, he was able to strike deep into the heart of al-Andalus during the expedition of 881. He negotiated an alliance with Pamplona and together the two kingdoms fought for control over Rioja. King Alfonso moved his capital from Oviedo to León, after which the kingdom was usually known as "Asturias and León". He may have been the first king of the new dynasty to use the title “Emperor”, although Payne points out that the sources are not unequivocal on this point. "Adefonsus rex" confirmed donations to Santiago de Compostela by charter dated 17 Aug 883, subscribed by "Exemena regina…". "Adefonsus rex et Exemena regina" donated property near Lanzada which had belonged to "Hermegildus filius Petri et uxor sua Yberia", who has rebelled against the king, to Santiago de Compostela by charter dated 24 Jun 886, subscribed by "Exemena regina, Garsea, Hordonius, Froila". "Adefonsus rex et Exemena regina" donated "ecclesiam sce. Marie…in villa…Arenosium" to Santiago de Compostela by charter dated 25 Jul 893, subscribed by "Exemena regina, Veremudus, Garsia, Ordonius, Froila, Gundisaluus". Conde Witiza rebelled against him but the rebellion was crushed in 895 with the help of conde Hermenegildo Gutiérrez. "Adefonsus rex" donated property to Sahagún by charter dated 28 Apr 909, subscribed by "Vimara Froilani, Garsea, Ordonius, Gundisalvus, Froila, Ranimirus, Dunninus". He revived Visigothic court forms, such as the traditional right of royal consecration.
The Crónica de Sampiro, as reproduced in the Historia Silense, records the death of King Alfonso at Zamora, and his burial "Oeto...sub aula sancte Marie Dey genetricis" after reigning for 44 years, the following passage recording the succession of his son “Era DCCCCXLVIII”. The Crónica de Sampiro (interpolated, España Sagrada edition) records that “uxore sua Xemena regina” was buried with her husband “Astoricæ” but that their bodies were transferred to “Oueto...sub aula sancte Marie Dey genitricis”. On his death, his territories were divided between his three sons. 1