On 19 December 1820 Moncrieff presided at the Pantheon meeting, which passed resolutions in favour of a petition to the Crown for the dismissal of the Tory ministry of Lord Liverpool. On 22 November 1826 he was elected Dean of the Faculty of Advocates, his friend Francis Jeffrey making way for him, though his senior, gracefully ceding his claim in favour of his friend. In 1828, following the custom of the bar that no criminal should be undefended, he acted for the "resurrectionist" William Burke. In March 1829 he spoke at a large meeting in Edinburgh in favour of Catholic Emancipation.