Thomas Mitchell was born in Craigend, now part of Glasgow, Scotland on June 15, 1792. He worked in his uncle's colliery (coal mine) business. He joined the British Army in 1811 with the rank of lieutenant. He fought in the Peninsula War in Spain against Napoleon. As well as actual fighting, he worked as a surveyor and made maps. After the war he made maps of the battlefields. He was made a Major in 1826, but was then put on half pay.