Formed within months of the French dissolution of the Electorate of Hanover in 1803, the KGL was constituted as a combined arms corps by the end of the year.

[2] [3][4] The same year, Major Colin Halkett and Colonel Johann Friedrich von der Decken were issued warrants to raise a corps of light infantry, to be named The King's German Regiment.

On 19 December 1803, Halkett's and von der Decken's levies were combined as a basis of a mixed corps (includes all arms: mounted, infantry, artillery) renamed the King's German Legion.

Although the legion never fought autonomously or as a single unit, its units participated in campaigns in Hanover, Pomerania, Copenhagen and Walcheren, the Peninsular War under General Sir John Moore; and the retreat to Corunna; the Peninsular Campaign under the Duke of Wellington, including the battles of Bussaco, Barrosa, Fuentes de Onoro, Albuera, Ciudad Rodrigo, Salamanca, Garcia Hernandez, Burgos, Venta del Pozo, Vittoria, San Sebastian, Nivelle, Orthez, Sicily, and the eastern parts of Spain, Northern Germany and Göhrde.

As the 5th Line Battalion under Oberst Ompteda was on its way to reinforce the defenders of Haye Sainte, the French cavalry attached to Jean-Baptiste Drouet, Comte d'Erlon's Corp I rode them down; only a few of the intended relievers survived.

[15] According to the historian Alessandro Barbero, the King's German Legion "had such a high degree of professionalism that it was considered equal in every way to the best British units.