Perhaps no figure of England’s Civil Wars and Interregnum (1642-1660) is more deserving of study than Edward Sexby (c. 1616 – d. 1658). From April 1647 to July 1657 Sexby was, in succession, an ‘agitator’ (or agent) put forward from the Long Parliament’s New Model Army to communicate soldierly grievances, and larger concerns over England’s future; an intermediary between Oliver Cromwell and John Lilburne in the forging of a New Model and Leveller alliance directed against Charles I; an army officer and intelligencer for the English Republic or Commonwealth; and lastly, a conspirator against Oliver Cromwell’s Protectorate. Scholars of England’s 1640s and 1650s have long commented on parts of Sexby’s story, but few scholars have examined his work from beginning to end as a career and considered its impact. This dissertation seeks to fill that important historiographical gap. The chapters of this study provide a narrative and an analysis of a career that is shown to have been consequential, inasmuch as Sexby was a figure behind some of the momentous events that occurred in England between the spring of 1647 and the spring of 1657. This study reveals that Sexby’s work helped to produce discussions in the New Model Army in the fall of 1647 of proposed changes to England’s constitution, the New Model and Leveller alliance of the fall of 1648 that was a step towards Charles’ execution and founding of the Commonwealth, and the second Protectorate parliament’s offer in the spring of 1657 to restore England’s monarchy with Oliver Cromwell as king. Three times in his career, Sexby’s work touched on the very constitution and future of England.