King Amlaíb granted the monks of Furness the right to elect the Bishop of the Isles, and it appears that Wimund was elected to the see during the time of Thurstan (II), Archbishop of York. Thurstan died in early 1140, so that Wimund became Bishop of the Isles in the period 1134–1140. This was a very rapid rise for a young man of apparently obscure origins.
However, as William of Newburgh tells us later, Wimund in time claimed to be the son of the Mormaer of Moray. William, and some later writers, doubted Wimund's claims. Modern historians have been more inclined to take this claim seriously. Some have proposed that Wimund was a son of Óengus of Moray (died 1130), grandson of King Lulach mac Gille Coemgáin. However, his link with Cumbria has led to the supposition that Wimund was an illegitimate son of William fitz Duncan, son of King Donnchad mac Maíl Coluim. William held extensive lands in Cumbria through his mother, Octreda, daughter of Cospatrick of Northumbria, and is believed to have been Mormaer or Earl of Moray between Óengus's death in 1130 and his own death in 1147.
Wimund's bishopric of the Isles had its seat on the Isle of Skye. The ruins of Snizort Cathedral, dedicated to Columba, are still visible near Skeabost. William of Newburgh writes that Wimund, "[n]ot content with the dignity of his episcopal office, he next anticipated in his mind how he might accomplish great and wonderful things; for he possessed a haughty speaking mouth with the proudest heart."
However, Wimund's father, if he was indeed the son of William fitz Duncan, was alive for the first seven years at least of his time as a Bishop of the Isles. So long as his father was alive, Wimund need hardly "[feign] himself to be the son of the earl of Moray and that he was deprived of the inheritance of his fathers by the king of Scotland" as William says. But William may be anticipating himself; Wimund's first conflict was not with his uncle King David, but with a fellow bishop, and there is no reason to suppose that these two conflicts were linked.
During Wimund's episcopate, or shortly before its beginning, Gille Aldan was consecrated Bishop of Whithorn, probably by the agreement of Fergus of Galloway and Archbishop Thurstan, and with the approval of Pope Honorius III. The lands of the recreated Bishopric of Whithorn had probably been subject to the Bishops of the Isles, and for rival bishops to employ armed force to drive off their rivals was hardly unknown. Thus, rather than to gain his inheritance, Wimund's struggle with Gille Aldan was apparently an attempt to prevent his bishopric being partitioned in favour of a rival.
Wimund was a charismatic man standing taller than most of those around him and of athletic build, a fiery speaker, he enticed a band to join him in not only in taking back his "inheritance", but in exacting revenge on the Scots. It was then that "..he began his mad career throughout the adjacent islands; and became, like Nimrod, a mighty hunter..". The fisher of men had become the hunter of men. He then began to invade provinces of Scotland "wasting all before him with rapine and slaughter". When royal Scottish troops tried to stop him, he would retreat to the sea or hide in remote forests, from which he would later re-emerge to continue his campaign of destruction.
Wimund ran in to trouble when he demanded tribute payments from a Scottish bishop; the Scottish bishop said "God's will be done; but from my example, no one bishop shall ever become tributary to another." The bishop gathered a small number of supporters to face Wimund's larger band and, like the story of David and Goliath, he threw a hand axe at Wimund and struck him down in the first blow of the battle; this emboldened the Scottish defenders who were able to kill vast numbers of Wimund's band, driving the rest from Scotland and forcing Wimund to "fly".
Wimund, recovered from the axe wound, and recovering the remainder of his forces, continued to ravage the coasts and islands of Scotland. The King of Scotland was forced to act and, knowing that force alone would not defeat a crafty and proud enemy like Wimund, devised a plan of placation. He granted Furness and a province to Wimund in order to plant a false sense of security, to instill in the proud and self-important Wimund the sense that he really was the lie he had for so long told, a nobleman and lord. Thus, with his guard let down, Wimund toured his new province while a number of enemies inside his camp "who were unable to endure either his power or his insolence" laid a trap for him. "Obtaining a favourable opportunity, when he was following slowly, and almost unattended, a large party which he had sent forward to procure entertainment, they took and bound him, and as both eyes were wicked, deprived him of both; and, providing against all future excess, they made him an eunuch for the sake of the kingdom of Scotland, not for that of Heaven.
Captured, blinded, and castrated by his enemies, he spent the rest of his life at the monastery at Byland Abbey, a willing storyteller for those who would listen. In regard to the Scottish Bishop who took him down with an axe, Wimond said "..with much pleasantry, boastingly.. that God alone was able to vanquish him by the faith of a simple bishop." Unrepented to the end, of his enemies who blinded and castrated him, he said that had he even the eye of a sparrow, "his enemies should have little occasion to rejoice at what they had done to him."