The Admonition Controversy (1572–1577), largely between Thomas Cartwright (1534/ 5–1603) and John Whitgift (1530–1604) has proven fecund ground for intellectual historians analysing the religious dimension to early-modern political ideas. This paper argues that the religious dimension of Cartwright’s mixed constitutionalism needs better explanation, rather than just noting that his ecclesiastical mixed constitutionalism (Presbyterianism) mirrors his political mixed constitutionalism. This paper tracks Cartwright’s progressive, dialogical unfolding of his mixed constitutionalism in response to Whitgift’s attempt to derive episcopacy from the fact of English monarchy, effectively discrediting the Admonition to Parliament (1572). Furthermore, the essay outlines how the Cartwright–Whitgift debate led Cartwright to emphasise a parliamentarist mixed constitution when most of his contemporaries, especially the more famous mixed constitutionalist, Thomas Smith, portrayed the English parliament leaning noticeably towards the monarch. This analysis accepts that religious polemic was a major driving force in the normalisation of parliamentarism, yet seeks to show exactly how this worked out in one of the most important church–state disputes in Elizabethan England.