Parker began his law studies in the office of Rufus G. Emory. After being admitted to the bar in 1823, Parker became a successful criminal defense attorney.[6] On July 5, 1832, Parker was appointed District Attorney for Suffolk County by Governor Levi Lincoln Jr.[7][8]
During his tenure as DA, Parker tried Abner Kneeland multiple times for blasphemy.[9] Kneeland's 1838 conviction was the last time a court in the United States jailed a defendant for blasphemy.[10] In 1842, Parker was the prosecutor in Commonwealth v. Hunt, a case which ruled that labor unions were legal.[11] In 1844 he tried Abner Rogers Jr. for the murder of Charles Lincoln. Rogers was acquitted by reason of insanity.[12]
In 1846 he prosecuted Albert Tirrell for the murder of Maria Bickford, a prostitute he left his wife to be with. Tirrell's attorney Rufus Choate presented the defense that Tirrell could have murdered Bickford while sleepwalking. Tirrell was found not guilty. It was the first time a sleepwalking defense was successful in a murder case.[13]
In 1849, Parker tried Washington Goode, an African-American sailor, for the murder of Thomas Harding. Parker's case was largely circumstantial, but Goode was found guilty and hanged on May 25, 1849.[14]
^"Monthly List Of Marriages And Deaths". The Emerald. January 2, 1908.
^City Council, Boston (Mass. ). (1903). Boston Marriages, 1751-1809. Documents of the City of Boston, Volume 4, Issues 101-160. Retrieved July 26, 2019.
^"Trial of Abner Rogers jr., for the murder of Charles Lincoln, before the Supreme Court of Massachusetts at Boston, in January 1844, and acquittal by reason of insanity". The Western Law Journal. January 2, 1908.
^{{Rogers, Alan. "Under Sentence of Death": The First Effort to Abolish the Death Penalty, Murder and the Death Penalty in Massachusetts. (Amherst & Boston: University of Massachusetts Press, 2008}}
^"Trial of Professor Webster". Boston Cultivator. April 6, 1850.
^"Miscellaneous Intelligence". The Monthly Law Reporter. March 1852.