After leaving the bench, Parker resumed practicing law in Albany, and founded a partnership that included former judge Edwin Countryman and Parker's son Amasa J. Parker Jr.  Among his well known cases was his successful argument to the United States Supreme Court that national banks were subject to state taxation.

Parker was one of the founders of the Albany (New York) Law School in 1851, and he was a member of the school's faculty for over twenty years.
 He was the unsuccessful Democratic candidate for Governor of New York twice, losing to Republicans John Alsop King in 1856 and Edwin D. Morgan in 1858.
 During the administrations of New York's Democratic governors, Parker declined several offers to reappoint him to the bench, and during the presidential administration of James Buchanan he declined appointment as United States Attorney for the Southern District of New York.

During the period before the American Civil War, Parker remained loyal to the Democratic Party and advocated a moderate course in the hope that concessions on the slavery issue would avoid bloodshed.
In 1861, he was the permanent chairman of the state Democratic convention.
 Once the war started, he supported the Union, but argued against what he saw as the excesses of the Lincoln administration.
 In 1864, he successfully argued the case of Palin v. Murray in Greene County, obtaining a judgment for the plaintiff on the grounds of false imprisonment by federal authorities.
 The case was later moved to the federal courts, where it was decided in Palin's favor by the U.S. Supreme Court in 1869.

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.

Parker was a longtime promoter of higher education, and served as a trustee or board of governors member of several institutions, including Union College, Cornell University, Albany Medical College, and the Albany Female Academy.
 While serving in the Assembly in 1834, Parker advocated the creation of a state hospital for the insane.
 When the facility was finally established as the Hudson River State Hospital in the 1860s, Parker was appointed to its board of trustees, and he served until 1881.
After leaving the bench, Parker resumed practicing law in Albany, and founded a partnership that included former judge Edwin Countryman and Parker's son Amasa J. Parker Jr.  Among his well known cases was his successful argument to the United States Supreme Court that national banks were subject to state taxation.
Parker was one of the founders of the Albany (New York) Law School in 1851, and he was a member of the school's faculty for over twenty years.  He was the unsuccessful Democratic candidate for Governor of New York twice, losing to Republicans John Alsop King in 1856 and Edwin D. Morgan in 1858.  During the administrations of New York's Democratic governors, Parker declined several offers to reappoint him to the bench, and during the presidential administration of James Buchanan he declined appointment as United States Attorney for the Southern District of New York.
During the period before the American Civil War, Parker remained loyal to the Democratic Party and advocated a moderate course in the hope that concessions on the slavery issue would avoid bloodshed. In 1861, he was the permanent chairman of the state Democratic convention.  Once the war started, he supported the Union, but argued against what he saw as the excesses of the Lincoln administration.  In 1864, he successfully argued the case of Palin v. Murray in Greene County, obtaining a judgment for the plaintiff on the grounds of false imprisonment by federal authorities.  The case was later moved to the federal courts, where it was decided in Palin's favor by the U.S. Supreme Court in 1869.
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.
Parker was a longtime promoter of higher education, and served as a trustee or board of governors member of several institutions, including Union College, Cornell University, Albany Medical College, and the Albany Female Academy.  While serving in the Assembly in 1834, Parker advocated the creation of a state hospital for the insane.  When the facility was finally established as the Hudson River State Hospital in the 1860s, Parker was appointed to its board of trustees, and he served until 1881.
Amasa J. Parker