 Promoted lieutenant in 1804, and captain in 1806, he served in the Light Division throughout the Peninsular War.Fergusson was present at the battles of Roliça, Vimeiro, (where he sustained wounds), and Corunna, where his regiment formed part of the reserve.Fergusson accompanied the 43rd on the 1809 Walcheren Expedition before returning to the Peninsula, experiencing action at River Côa, Bussaco, Sabugal, Fuentes de Onoro, as well as Ciudad Rodrigo and Badajoz, where he formed part of the 43rd's storming parties, receiving wounds both times.