Tomkinson was gazetted to the 16th Light Dragoons as a cornet in December 1807, joining his regiment the following April. During the Peninsular War (1809-1813) he was seriously wounded at the crossing of the Douro on 11May 1809 but recovered to see action at the battles of Busaco (1810), Redhina (1811), Fuentes de Oñoro (1811), El Bodón (1811), Salamanca (1812) and Vittoria (1813).