On 8 June 1809 (aged 16), Thomas enlisted as an Ensign in the 2nd Battalion of the 48th (Northamptonshire) Regiment of Foot in the British Army. On 17 June 1811, he was promoted to the rank of lieutenant. He served in the Peninsular War and was awarded the Military General Service Medal, with clasps for Busaco (1810), Albuera (1811), Ciudad Rodrigo (1812), Badajoz (1812), Salamanca (1812), Vittoria (1813), Orthes (1814) and Toulouse (1814).