On the outbreak of the Civil War supported Parliament and served on the Navy Committee (1646-52) and the Council of State (1651-53). He also signed the death warrant of the king in 1649.
Carew retained his radical political ideas and became a member of the Fifth Monarchist group that sought the abolition of tithes, an increase in the help for the poor and the release of debtors from prison. He also opposed to the dictatorial rule of Oliver Cromwell and was imprisoned twice during the Commonwealth.
On the Restoration Carew was an obvious target for the Royalists. He refused to flee the country and was therefore like other Regicides arrested and brought to the Tower of London. At his trial in October 1660 he was found guilty of treason and was hung, drawn and quartered.