In May 1596, Carew took part in the expedition to Cadiz with the Earl of Essex, in 1597 in the expedition to the Azores and in the same year during the 3rd Spanish Armada invasion attempt. Soon after he was elected Member of Parliament for Queenborough, In 1598 he went to France for a short time as ambassador to the court of King Henry IV in the company of secretary Cecil. He was appointed treasurer at war to Essex in Ireland in March 1599, and on the latter's sudden departure in September of the same year, leaving the island in disorder, Carew was appointed a lord justice.