Uxbridge ordered his two brigades of British heavy cavalry—formed unseen behind the ridge—to charge in support of the hard-pressed infantry. The 1st Brigade, known as the Household Brigade, commanded by Major-General Lord Edward Somerset, consisted of guards regiments: the 1st and 2nd Life Guards, the Royal Horse Guards (the Blues), and the 1st (King's) Dragoon Guards. The 2nd Brigade, also known as the Union Brigade, commanded by Major-General Sir William Ponsonby, was so called as it consisted of an English (the 1st or The Royals), a Scottish (2nd Scots Greys), and an Irish (6th or Inniskilling) regiment of heavy dragoons.

The Household Brigade crossed the crest of the Anglo-allied position and charged downhill. The cuirassiers guarding d'Erlon's left flank were still dispersed, and so were swept over the deeply sunken main road and then routed. Continuing their attack, the squadrons on the left of the Household Brigade then destroyed Aulard's brigade. Despite attempts to recall them, they continued past La Haye Sainte and found themselves at the bottom of the hill on blown horses facing Schmitz's brigade formed in squares.

Napoleon promptly responded by ordering a counter-attack by the cuirassier brigades of Farine and Travers and Jaquinot's two Chevau-léger (lancer) regiments in the I Corps light cavalry division. Disorganized and milling about the bottom of the valley between Hougoumont and La Belle Alliance, the Scots Greys and the rest of the British heavy cavalry were taken by surprise by the countercharge of Milhaud's cuirassiers, joined by lancers from Baron Jaquinot's 1st Cavalry Division.

As Ponsonby tried to rally his men against the French cuirassers, he was attacked by Jaquinot's lancers and captured. A nearby party of Scots Greys saw the capture and attempted to rescue their brigade commander. The French lancer who had captured Ponsonby killed him and then used his lance to kill three of the Scots Greys who had attempted the rescue. By the time Ponsonby died, the momentum had entirely returned in favour of the French. Milhaud's and Jaquinot's cavalrymen drove the Union Brigade from the valley. The result was very heavy losses for the British cavalry. A countercharge, by British light dragoons under Major-General Vandeleur and Dutch–Belgian light dragoons and hussars under Major-General Ghigny on the left wing, and Dutch–Belgian carabiniers under Major-General Trip in the centre, repelled the French cavalry.