Somerled was born around 1113 in Morven, Argyleshire. He was the son of Gillebride Mac Gille Adomnan and a Viking woman. Although there is some contention on his ancestry, his father was apparently either of the Royal line of Dalriada, Gall Gael (which is Cruithni or Pict) or both. Somerled's name means 'summer wanderer', a name used by his contemporaries to describe the Vikings. For Somerled, it was a name that prophecized his life -and the combination of bloodlines, at least in Somerled's case, proved itself powerful, as he later forged a permanent spot for himself in the history of the Isles and Scotland.

Sometime in Somerled's early youth, the Lochlans and the Fingalls (Clans or tribes) expelled Somerled's family from their home. They took refuge in Ireland, where Gillebride managed to persuade the Colla (an Irish tribe) to assist him in the recovery of his possessions or holdings. A large force of approximately 500 men accompanied the family home. The mission was a failure, however, and his father either died in the battle or soon afterwards.

Somerled lived for a while in the caves of his homeland, fishing and hunting for his survival. Slowly he grew into manhood and became, according to the accounts; "A well tempered man, in body shapely, of a fair and piercing eye, of middle stature and quick discernment." During this period of his life several things happened in quick succession which made Somerled a man of stature.

In one story, Somerled put himself at the head of the inhabitants of Morven and attacked the Norwegians. He was successful, and recovered his family's lands at the same time. He then was master of Morven, Lochaber and northern Argyle. Soon after this he conquered the southern portions and pronounced himself Thane or Regulus of Argyle. This happened at about the same time as David the First's war with the Norwegians, which took place in 1135, so Somerled may have received these lands in a grant from the King.

His newfound power greatly increased his standing, but it also drew the attention of his neighbors, the Vikings in the Isles (the Isle of Skye, the Isle of Man and that general area). Somerled, however, still did not have the force required to take on the Olaf the Red, the Viking Lord of the Isles. Instead he chose to woo his enemy for the hand of his daughter, Ragnhild. Eventually he succeeded (some say by trickery) in obtaining Olaf's daughter's hand and the two were married in approximately 1140.

For the next fourteen years Somerled and Ragnhild lived in relative peace and started raising a family. Raginald gave him three sons, Dugall, Reginald, and Angus. These sons joined his son by a previous marriage, Gillecallum.

In 1154, Olaf (Olave in some stories) was murdered by his nephews who quickly took control of the northern half of the Kingdom of the Isles. Olaf's son, Godfred (or Godfrey) heard of the events and returned from Norway, quickly regaining possession of the entire Kingdom. But Godfrey was a tyrant, and the Islemen soon revolted against his leadership. Some of the chieftans of the Isles appealed to Somerled for help. He joined them and defeated Godfrey, in the process taking the southern half of the Kingdom for himself. About two years later Godfrey and Somerled again went to war, this time Somerled was using new ships with a rudder and Godfrey was defeated again. Somerled became King of the Isles in about 1156.

At about the same time, Somerled was also campaigning in Scotland to a small degree and this in combination with his new title as King of the Isles drew the attention of its King. King Malcolm IV of Scotland was concerned over Somerled's growing power and dispatched an army to Argyle. In 1160, after a battle the two Kings reached an understanding and there was again peace. This peace was short lived however, as in late 1163, after being continually insulted by Malcolm and his ministers, Somerled led an army against Scotland.

The King of the Isles sailed up the Clyde with 164 galleys and 15,000 troops to Greenock. He landed at the Bay of St. Lawrence and marched on Renfrew. There are two popular stories about what occurred in Scotland. In one version, a bribed nephew murdered Somerled and the army of the Isles dispersed and went home. In the other version of the story, battle was joined between the Scots and the men of the Isles and Somerled was killed. His son Gillecallum, his heir, also died during the battle. Now without a leader, the army from the Isles dispersed and went home. In either case Somerled died in Scotland in very early 1164.

Somerled is generally credited with breaking the power of the Vikings in the Isles as his descendants remained Kings of the Isles for centuries after his death. One of Somerled's grandsons, a Donald, is also considered the ancestor of the Clan Donald, for his sons were the first to carry the name MacDonald.