King Robert won his first small success at Glen Trool, where he ambushed an English force led by Aymer de Valence, attacking from above with boulders and archers and driving them off with heavy losses. He then passed through the moors by Dalmellington to Muirkirk, appearing in the north of Ayrshire in early May, where his army was strengthened by fresh recruits. Here he soon encountered Aymer de Valence, commanding the main English force in the area. In preparing to meet him he took up a position on 10 May on a plain south of Loudoun Hill, some 500 yards wide and bounded on either side by deep morasses.

Valence's only approach was over the highway through the bog, where the parallel ditches the king's men dug outwards from the marsh restricted his room for deployment, with the ditches in front of the Scots impeding him still further, effectively neutralizing his advantage in numbers. Valence was forced to attack along a narrowly constricted front upwards towards the waiting enemy spears. It was a battle reminiscent in some ways of Stirling Bridge, with the same 'filtering' effect at work.

A frontal charge by the English knights was stopped by the king's spearmen militia, who effectively slaughtered them as they were on unfavourable ground, thus the militia soon defeated the knights. As the king's spearmen pressed downhill on the disorganised knights, they fought with such vigour that the rear ranks of the English began to flee in panic. A hundred or more were killed in the battle, while Aymer de Valence managed to escape the carnage and fled to the safety of Bothwell Castle.