Caxias kept up his family's tradition by joining the military service at age 14, and within a year he was promoted to second lieutenant. At 21 he was appointed adjutant of the battalion of the emperor. Caxias first displayed his military skills during the war of independence (1823) in the state of Bahia. On April 7, 1831, the emperor Pedro I abdicated, and Caxias organized a battalion to police Rio de Janeiro until a new government was formed, one that featured his father as co-regent. After becoming governor of the state of Maranhão in 1837, Caxias was elected to Brazil's House of Deputies in 1840.

Caxias resumed his military career when he was asked to pacify local disturbances in the states of São Paulo, Minas Gerais, and Rio Grande do Sul (1841–45), where a separatist revolt was in progress; in 1852 he was appointed to lead the Brazilian troops that helped overthrow the Argentine dictator Juan Manuel de Rosas. In 1855 Caxias was named minister of war, a post he held for many years; he also served as president of the council of ministers twice (1866 and 1875–78). In 1867 Caxias was given command of the army, which had suffered two years of reverses during the Paraguayan War (1864–70). After his capture of Asunción in 1869, he was named duke of the empire and was considered the champion of the conservatives in Brazil. He is the official patron of the Brazilian army.