The Duke of Somerset brought his troops, cavalry and guns to the area, with naval support for his sixteen thousand men advancing along the beach. The Scots, numbering thirty-six thousand, held the better position behind the river, but lacked experience and effective cavalry. They were led by James Hamilton, 2nd Earl of Arran, who fatally misinterpreted an English retreating manoeuvre. Hamilton ordered his men across the river in a full-out charge, in doing so lost his advantage. Somerset capitalised on this serious blunder with use of his artillery.
By the end of the battle Somerset's men had slain fifteen thousand Scots and captured fifteen hundred more, while losing only five hundred of their own men.
While Black Saturday was a disaster for the Scots, it was a dubious victory for the English since it pushed Queen Mary further away from Prince Edward.