In 1867, Parker was a delegate to the New York State Constitutional Convention of 1867. He was heavily involved in committees that considered reorganization of the state courts, and successfully advocated abolition of the chancery courts, and the vesting of both law and equity powers in the same courts. In 1868, Parker took on a rare criminal defense case and obtained an acquittal for George W. Cole on the grounds of temporary insanity after Cole shot and killed L. Harris Hiscock. In the early 1870s, Parker declined the large retainer offered to defend William Tweed during Tweed's trials for corruption. In his later years, Parker compiled, edited and published six volumes of reports on criminal cases, and took part with several colleagues in publishing a revised edition of New York's state statutes.