He obtained a commission in the 43rd (Monmouthshire) Regiment of Foot (Light Infantry) (later the 1st Battalion, Oxfordshire and Buckinghamshire Light Infantry) in 1806, and he was promoted to lieutenant a few months later. He saw active service for the first time in the Mediterranean and at Copenhagen, in 1806 and 1807 respectively. The 43rd was one of the earliest arrivals in Spain when the Peninsular War broke out, and Brown was with his regiment at Vimeiro, and in the Corunna retreat. Later in 1809 the famous Light Division was formed, and with Craufurd he was present at all the actions of 1810–1811, being severely wounded at Talavera; he was then promoted captain and attended the Staff College at Great Marlow until (late in 1812) he returned to the Peninsula as a captain in the 85th. With this regiment he served under Major-General Lord Aylmer at the Nivelle and Nive, his conduct winning for him the rank of major.