Brotherton entered the 2nd or Coldstream Guards as an ensign in 1800, was promoted to lieutenant and to captain in 1801, and transferred to the 3rd or Scots Fusilier Guards in 1803. With the guards he served under Ralph Abercromby in Egypt in 1801, and under Lord Cathcart in Hanover in 1805. On 4 June 1807 he exchanged into the 14th Light Dragoons. With it he served almost continuously in the Peninsula from 1808 to 1814. He was in Sir John Moore's retreat to Corunna; he was present at Talavera, at the actions on the Côa, at Bussaco, Fuentes de Oñoro, Battle of Salamanca, where he was wounded, Vitoria, the Pyrenees, the Nivelle, and the Nive, where he was severely wounded and taken prisoner.