From 1808 he served as a military surgeon with the 91st Regiment of Foot, serving during the Walcheren Campaign within the Napoleonic Wars. This action saw huge injuries, and Maclagan's experience would have increased exponentially from this experience. From 1810 to 1813 he served in various military campaigns, including the attack on Badajos during the Peninsular War, the Battle of Salamanca, the Battle of Vittoria, the Battle of the Pyrenees, the Battle of Nivelle and the Battle of Nive.