Grimes returned at the end of the year, and became involved in the deposition of Governor William Bligh in the Rum Rebellion on 26 January 1808. He was one of the members of a committee formed to examine the administration of Bligh, and was appointed acting judge-advocate. In this capacity, he sat at the token trial of John Macarthur. Grimes realised, however, that his lack of legal training made it difficult for him to act effectively in his judicial position and he resigned on 5 April 1808. He was sent to England with dispatches that same month.