After the death of his uncle Gille Brigte in 1185, Lochlann went about to seize the land of Gille Brigte's heirs. In this aim he had to defeat the men who would defy his authority in the name of Gille Brigte's heir. He seems to have done so, defeating the resistors, who were led by men called Gille Pátraic and Henric Cennédig. Yet resistance continued under a warrior called Gille Coluim of Galloway.
Lochlann's aims moreover encouraged the wrath of a more important political figure than any of the above. King Henry II of England was outraged. A few years before Gille Brigte's death, Henry had taken his son and successor Donnchad as a hostage. Hence Henry was the patron and protector of the man Lochlann was trying to disinherit. When King William of Scotland was ordered to visit Henry in southern England, William was told that Lochlann must be stopped. However, William and Lochlann were friends, and so in the end Henry himself brought an army to Carlisle, and threatened to invade unless Lochlann would submit to his judgment. Lochlann did so. As it transpired, Lochlann kept most of Galloway, and Donnchad was given the new "Mormaerdom" of Carrick in compensation.
More than any previous Lord of Galloway, he was the loyal man and vassal of the King of Scotland. After all, he owed his lands to the positive influence of King William. Whereas Lochlann's grandfather, Fergus had called himself King of Galloway, Lochlann's favorite title was "Constable of the King of Scots".
Lochlann had led William's armies north into Moireabh against the pretender Domnall mac Uilleim, who claimed the Scottish throne as a grandson of King Donnchad II of Scotland. Lochlann defeated him in 1187 at the Battle of Mam Garvia, a mysterious location probably near Dingwall.
Lochlann, unlike his uncle Gille Brigte, welcomed French and English colonization into his eastern lands. In this, he was following his overlord, King William I of Scotland. Of all the Lords of Galloway, Lochlann is the least mentioned in the Gaelic annals, suggesting that he had lost touch somewhat with his background in the world of greater Irish Sea Gaeldom.
In 1200, he was in the company of King William in England, who was giving homage to the new king, John. Lochlann used the opportunity to make legal proceeding in Northampton regarding the property claims of his wife, Helena, daughter and heiress of Richard de Morville. It was here that he met his death and was buried. Lochlann and Helena had a son Alan, who succeeded to Galloway.