New Model Army commanded by Sir Thomas Fairfax and Oliver Cromwell.
The Royalists did not see Fairfax's position until they reached the village of Clipston just over a mile north of Naseby ridge. It was clearly impossible for the Royalists to withdraw to their original position without being attacked by the Parliamentarian cavalry while on the line of march and therefore at a disadvantage. Rupert deployed the army to its right, where the ground appeared to be more favourable for his own cavalry and prepared for battle.
The Royalist army right wing consisted of between 2,000 and 3,000 cavalry under Rupert and his brother Prince Maurice. The centre was organised as three infantry brigades commanded by Lord Astley, with a regiment of horse under Colonel Howard in support. On the left were 1,500 "Northern Horse" commanded by Sir Marmaduke Langdale. The King commanded a small reserve, consisting of his own and Prince Rupert's regiments of foot and his Lifeguard of Horse.
Fairfax had drawn up the New Model Army on the ridge a mile north of Naseby, with some of it behind the crest on the reverse slope. Commissary-General Ireton's wing of five and a half regiments of cavalry was on the left. The infantry under Sergeant-Major General Sir Philip Skippon was in the centre with five regiments in the front line and three in support. A forlorn hope of 300 musketeers was deployed to the front, and two companies of Colonel Edward Harley's regiment were in reserve. A Parliamentarian engraving of the battle shows 11 pieces of artillery inbetween the infantry regiments but they played little part in the battle; their first salvos went high, and the Royalist and Parliamentarian infantry were subsequently too closely engaged for the guns to be used. Cromwell's wing, with six and a half regiments of cavalry, was on the right.
The new Model Army outflanked the Royalist left, but their own left flank rested, like the Royalists' right flank, on the Sulby Hedges. At the last minute, as the Royalists began to advance, Cromwell sent a regiment of dragoons under Colonel John Okey into the Sulby Hedges, where they could fire into the flank of Rupert's cavalry.
The Royalist centre advanced first, Rupert keeping his own wing of cavalry in hand so that the horse and foot could hit the enemy simultaneously.Both sides advanced ans hand to hand happened quickly - and initially the Parliamentarians were hard-pressed and forced back.
On the Parliamentarian left the New Model Army Horse under Ireton repulsed the Royalist Cavaliers and swung in to aid of the beleaguered Parliamentarian infantry. The Parliamentarian Horse were driven off by Royalist foote and Ireton himself was unhorsed and taken prisoner. At the same time, the second line of Royalist cavalry broke the rest of the Parliamentarian horsemen who broke and fled, allowing some proper Cavalier action as Rupert either failed or was unable to rally the Cavalier horsemen, who galloped off the battlefield in pursuit of the fleeing Parliamentarians.
Meanwhile the Parliamentarian right wing of horse under Oliver Cromwell and the Royalist Northern Horse faced each other, neither willing to charge to the aid of their infantry while the other could threaten their flank. Eventually after half an hour, the Royalist cavalry began to charge and Cromwell's troops moved to meet them, and after a brief contest routed them.
Cromwell had won with only half of his wing of Horse allowing him to send four divisions in pursuit and turn the rest against the left flank and rear of the Royalist Foote in the centre. At about the same time Okey's dragoons mounted their horses and charged from the Sulby hedges against the right wing of the Royalist infantry, as did some of Ireton's regiments which had partly rallied.
Some of the trapped Royalist infantry began to throw down their arms and call for quarter; others tried to conduct a fighting retreat. One regiment, apparently Prince Rupert's Bluecoats, stood their ground and repulsed all attacks.
Things were going badly for the Royalists, and the King was tempted to lead his Lifeguard of Horse to the rescue, but was prevented from doing so by a Scottish nobleman, the Earl of Carnwath, who swore at him "Would you go upon your death?" Seeing the King swerve away from the enemy, his Lifeguard also retreated in disorder for several hundred yards.
The Royalist Cavaliers under Rupert reached the Parliamentarian baggage, but struggled to capture it as the Parliamentarians had a unit acting as camp guards. Rupert instead rallied his men and led them back to the battlefield but it was by now too late to save the remnants of the Royalist infantry, so Rupert's cavalry rode off the field.