Sydenham was the fourth son of John Poyntz of Reigate, Surrey, and his wife Ann. He ran away from an apprenticeship to a London tradesman and became a mercenary, fighting for the Imperial Spanish army under Mansfeld during the Thirty Years' War and rising to the rank of Major General.
By 1647, Poyntz was drawn onto Parliament's side of the dispute between it and Oliver Cromwell's New Model Army. New Model Army commander Fairfax mistrusted Poyntz and placed spies in Poyntz's camp to foment distrust. Poyntz fled the country when London fell to Army troops. He wrote The Relation of Sydnam Poyntz 1624-1636, an account of his German experiences.
- Goldman, Lawrence, Oxford Dictionary of National Biography, Poyntz, Sydenham, D.N. Farr
- Full text of "The relation of Sydnam Poyntz, 1624-1636", archive.org
|Wikisource has the text of the 1885–1900 Dictionary of National Biography's article about Poyntz, Sydenham.|
- D. N. Farr, ‘Poyntz , Sydenham (bap. 1607)’, Oxford Dictionary of National Biography, Oxford University Press, Sept 2004; online edn, Oct 2006
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