John Belasyse had been made Governor of York and commander of Royalist forces in Yorkshire in January 1644. The job was harder than it sounded. Through that winter, Parliamentarian raiding parties had ranged across the county - north as far as Whitby, west to Bradford - and Belasyse moved his headquarters from York to Selby to better guard the southern approach to the city. In late March, reinforced by Royalist cavalry from Newark under Major-General George Porter, he and Porter went on the offensive against the Parliamentarian garrison at Bradford. The attack collapsed when Porter's cavalry were routed. Both commanders pulled back to their respective garrisons. The Parliamentarians, smelling blood, decided to come for Selby.

Lord Fairfax brought a contingent from Hull. His son, Sir Thomas Fairfax - who would later command the New Model Army and become the most famous Parliamentarian general of the war - joined him from Cheshire with cavalry and Lancashire infantry. Colonel John Lambert came in from Bradford. Sir John Meldrum brought troops from the Midlands Association. Combined, the Parliamentarians fielded approximately 1,500 horse and 1,800 foot - a force assembled across half of northern England in days. The plan was simple. The unfortified town had only the four road barricades to defend it. Hit three of them simultaneously with infantry, force the Royalists back into the town centre, then push the cavalry through the gaps. The fourth road was left as an apparent escape route, but the River Ouse closed it off on the far side.

On the morning of 11 April, Lord Fairfax led his regiment against the Ousegate barricade. Sir John Meldrum took his regiment down Gowthorpe Lane. Colonel Needham led the third regiment down Brayton Lane. Sir Thomas Fairfax waited with the Parliamentarian cavalry. Musket fire met the advancing infantry at each barrier. The fighting moved back and forth in attacks and counter-attacks - the Royalists trying to hold their barricades, the Parliamentarians grinding forward against fire from defenders who knew that losing the barriers meant losing the town. After more than an hour, the Royalists began to fall back toward the town centre, and the Royalist cavalry tried to screen the infantry retreat. That was when Sir Thomas Fairfax committed his horse. The Royalist cavalry broke and fled across the Ouse on a bridge of boats. Belasyse stayed to fight on, was unhorsed, and was taken prisoner.

Approximately 1,600 men of the Royalist garrison were taken captive, along with arms and ammunition. A few escaped on horse. The loss of Selby was a disaster for the Royalists in northern England. York was only 20 kilometres north and was now held by just two Royalist regiments. The Marquess of Newcastle, who had been holding County Durham against the Parliamentarians' Scottish allies, had to rush south to defend York. Eleven days after Selby fell, the Parliamentarians began the siege of York. That siege would draw Prince Rupert north with relief forces, and his arrival would lead, on 2 July 1644, to the Battle of Marston Moor - the largest battle of the Civil War and the one that effectively ended Royalist control of the north. Selby was the first move in that whole sequence: an unfortified market town, four roads, three regiments advancing together at dawn.