From "Debrett's Kings and Queens of Britain" by David Williamson, ISBN 0-86350-101-X, p. 21:
He succeeded his father as King of all England in 839. Before that, he had been sub-king in Kent, Essex, Sussex and Surrey since 825, so he was well versed in government. On succeeding to the larger kingdom, the sub-kingship was taken over by Athelstan who was either Ethelwulf's eldest son, or, perhaps more likely, his younger brother. Ethelwulf spent the greater part of his reign dealing with the Danish marauders whose raids were made in increasing numbers.
His first wife was Osburh (Osburga), daughter of Ealdorman Oslac of Hampshire, the royal cup-bearer, a descendant of Cerdic's nephew Wihtgar, who had been settled in the Isle of Wight. She bore him at least four sons and one daughter, Ethelswith, who was married in 853 to her father's ally Burhed, King of Mercia. Osburga seems to have died soon after this event and the sorrowing Ethelwulf resigned his kingdom to his son Ethelbald and went on a pilgrimage to Rome, taking with him his youngest son Alfred, a boy of some eight years, who had already been to Rome three years before.
They were well received by Pope Leo IV who administered the rite of confirmation to Alfred, an act mistakenly taken by Asser, King Alfred's biographer, to be a consecration to future kingship, which was hardly foreseeable as Alfred had three elder brothers living. Ethelwulf and Alfred stayed in Rome for a year and on the return journey stopped at the court of Charles the Bald, King of the Franks and Charlemagne's grandson. Charles had a daughter Judith, who could not have ben more than 12 or 13 at the time, and he gave her in marriage to Ethelwulf, the wedding being solemnized at Verberie-sur-Oise on 1 Oct 856. Ethelwulf returned home in good health but died over a year later on 13 Jan 858. He was buried first at Steyning in Sussex, but was later removed to Winchester.

Reigned 839-856 (abdicated). Under-king of Kent 825-839 and 856-858. Renowned for his military prowess, he reputedly defeated 350 viking ships (851). He reduced taxation, endowed the Church, made lay lands inheritable, and provided systems of poor relief.

Excerpt from the Anglo Saxon Chronicle:
AD 855. In this year heathen men (the Vikings) first took up their quarters over the winter in Sheppey. And in the same year king Æthelwulf chartered the tenth part of his land over all his kingdom, for the glory of God and his own eternal salvation: and in the same year went to Rome with great pomp, and dwelt there twelve months, and then returned home; and Charles, kin of the Franks, then gave him his daughter for queen; and after that he came to his people, and they were rejoiced thereat; and two years after he came from France, he died, and his body lies at Winchester, and he reigned 18 years and a half. And Æthelwulf was son of Egbert, Egbert of Ealhmund, Ealhmund of Eafa, Eafa of Eoppa, Eoppa of Ingild; Ingild was the brother of Ine, king of the West Saxons, who held the kingdom thirty-seven winters, and afterwards went to St. Peter's , and there gave up his life. And they were the sons of Cenred, Cenred of Ceowald, Ceowald of Cutha, Cutha of Cuthwine, Cuthwine of Ceawlin, Ceawlin of Cynric, Cynric of Cerdic, Cerdic of Elesa, Elesa of Esla, Esla of Giwis, Giwis of Wig, Wig of Freawine, Freawine of Frithogar, Frithogar of Brond, Brond of Baldag, Baldag of Woden, Woden of Frithuwald, Frithuwald of Frealaf, Frealaf of Frithowulf, Frithowulf of Finn, Finn of Godwulf, Godwulf of Geat, Geat of Tatwa, Tatwa of Beaw, Beaw of Sceldwa, Sceldwa of Heremod, Heremod of Itermon, Itermon of Hathra, Hathra of Hwala, Hwala of Bedwig, Bedwig of Sceaf, that is son of Noah; he was born in Noah's ark; Lamech, Methuselah, Enoch, Jared, Mlahel, Cainan, Enos, Seth, Adam, the first man and our father, that is Christ, Amen. And then Æthelwulf's two sons succeeded to the kingdom, Æthelbald to the kingdom of the West Saxons, and Æthelberht to the kingdom of the Kentish people, and to the kingdom of the East Saxons, and to Surrey, and to the kingdom of the South Saxons. And then Æthelbald reigned five years. Alfred, his third son, he had sent to Rome; and when pope Leo heard say that [Æthelwulf] was dead, he blessed Alfred as king, and held him to the episcopal hands, as his father Æthelwulf, in sending him thither, had requested.
Æthelwulf was the son of Egbert and a sub-king of Kent. He assumed the throne of Wessex upon his father's death in 839. The usual Viking invasions and repulsions common to all English rulers of the time characterize his reign, but the making of war was not his chief claim to fame. Æthelwulf is remembered, however dimly, as a highly religious man who cared for the establishment and preservation of the church. He was also a wealthy man and controlled vast resources. Out of these resources, he gave generously, to Rome and to religious houses that were in need.
He was an only child, himself, but had fathered five sons, by his first wife, Osburga. He recognized that there could be difficulties with contention over the succession. He devised a scheme which would guarantee (insofar as it was possible to do so) that each child would have his turn on the throne without having to worry about rival claims from his siblings. Æthelwulf provided that the oldest living child would succeed to the throne and would control all the resources of the crown, without having them divided among the others, so that he would have adequate resources to rule. That he was able to provide for the continuation of his dynasty is a matter of record, but he was not able to guarantee familial harmony with his plan. This is proved by what we know of the foul plotting of his son, Æthelbald, while Æthelwulf was on pilgrimage to Rome in 855. Æthelwulf was a wise and capable ruler, whose vision made possible the beneficial reign of his youngest son, Alfred the Great.