Seymour briefly fought as a volunteer in the Anglo-Persian War (1855–1857) and almost immediately afterwards, was at the Relief of Lucknow during the Indian Mutiny (1857–1858). He fought in Italy and Sicily, as a civilian volunteer, joining Giuseppe Garibaldi's Esercito Meridionale (Southern Army) as a private in 1860. At an early stage he assumed the rank of captain on the basis that along with his younger brother Lord Edward Seymour (1841–1865) he had co-commanded Volunteer Cavalry in England; both brothers assuming the rank of captain. He distinguished himself in the Battle of Volturnus (1860) in late 1860 and in other campaigns. Garibaldi later confirmed his rank of Captain despite the fact that Seymour was merely a civilian volunteer, and he assumed the name pseudonym "Captain Richard Sarsfield" in commemoration of a hero he had heard about when at Christ Church, Oxford.