In 1578, Carew was made captain in the royal navy and undertook a voyage with Sir Humphrey Gilbert. In 1579–1580, he led a regiment of Irish infantry, then a regiment of cavalry, during the Baltinglass and Desmond rebellions. On the death of his brother in the Battle of Glenmalure, from which fight he had been kept by his uncle Jacques Wingfield, he was appointed constable of Leighlin castle. Soon afterward he killed with his own hands several Irishmen suspected of his brother's killing and was censured by the government.