House party
After the death of Oliver Cromwell Sydenham became one of Richard Cromwell's council ; but in April 1659 he acted with Fleetwood, Desborough, and what was termed the Wallingford House party to force him to dissolve the third Protectorate Parliament. According to Ludlow, he was one of the chief agents in the negotiation between the army leaders and the republicans which led to Richard's fall. On the restoration of the Rump Parliament Sydenham became a member of the Committee of Safety on 7 May 1659 and of the Council of State just over a week later on 16 May, though he had conscientious scruples against taking the oath required from members of the latter. He was also given the command of a regiment of foot.