The Lord Protector of the Commonwealth of England, Scotland and Ireland was the title of the head of state during the Commonwealth (or, to monarchists, the Interregnum), following the first period when a Council of State held executive power. The title was held by Oliver Cromwell (December 1653–September 1658) and his son and designated successor Richard Cromwell (September 1658–May 1659) during what is now known as The Protectorate.
The 1653 Instrument of Government (republican constitution) stated that—
Oliver Cromwell, Captain-General of the forces of England, Scotland and Ireland, shall be, and is hereby declared to be, Lord Protector of the Commonwealth of England, Scotland and Ireland, and the dominions thereto belonging, for his life.
The replacement constitution of 1657, the Humble Petition and Advice, gave ‘His Highness the Lord Protector’ the power to nominate his successor. Cromwell chose his eldest surviving son, the politically inexperienced Richard. This was a non-representative and de facto dynastic mode of succession, with royal connotations in both styles awarded, even a double invocation 16 December 1653 - 3 September 1658 "By the Grace of God and Republic Lord Protector of England, Scotland and Ireland" and many other monarchic prerogatives, such as awarding knighthoods.
The younger Cromwell, who succeeded on his father's death in September 1658, held the position for only eight months before resigning in May 1659, being followed by the second period of Commonwealth rule until the Restoration of the exiled heir to the Stuart throne Charles II in May 1660.