He was killed by Polish Lancers at the battle of Waterloo.
Ponsonby's part in the battle and his death during it, has been remembered in detail, because it highlights some pertinent points about cavalry. The charge of the Scots Greys, which as their commanding officer he led, had been very successful against their initial objective, the disorganised French columns of d'Erlon's corps. But then flushed with this success, he allowed the charge to continue. This was an error, because it left his regiment open to a counter attack against tired men and blown horses. His other mistake was to mount a horse of less value than the best one in his stable. These two errors contributed to his death on an exhausted horse, which had became bogged down in mud near to enemy lines.