Admitted to the Massachusetts bar later that year, Curtis began his legal career. In 1834, he moved to Boston and joined the law firm of Charles P. Curtis, where he developed expertise in
admiralty law and also became known for his familiarity with
patent law. In 1836, Curtis participated in the Massachusetts "
freedom suit" of
Commonwealth v. Aves as one of the attorneys who unsuccessfully defended a slaveholding father. When New Orleans resident Mary Slater went to Boston to visit her father, Thomas Aves, she brought with her a young
slave girl about six years of age, named Med. While Slater fell ill in Boston, she asked her father to take care of Med until she (Slater) recovered. The
Boston Female Anti-Slavery Society and others sought a writ of
habeas corpus against Aves, contending that Med became free by virtue of her mistress's having brought her voluntarily into Massachusetts. Aves responded to the writ, answering that Med was his daughter's slave, and that he was holding Med as his daughter's agent. The Supreme Judicial Court of Massachusetts, through its Chief Justice,
Lemuel Shaw, ruled that Med was free, and made her a ward of the court. The Massachusetts decision was considered revolutionary at the time. Previous decisions elsewhere had ruled that slaves voluntarily brought into a free state, and who resided there many years, became free.
Commonwealth v. Aves was the first decision to hold that a slave voluntarily brought into a free state became free the moment he or she arrived. The decision in this freedom suit proved especially controversial in slaveholding southern states. As with his fellow Massachusettsan and Harvard graduate
John Adams, Curtis's willingness to serve as defense attorney for the Aves family did not necessarily reflect his personal or legal views, as shown by his later dissent in the 1857
Dred Scott decision. Curtis became a member of the
Harvard Corporation, one of the two governing boards of Harvard University, in February 1846. In 1849, he was elected to the
Massachusetts House of Representatives. Appointed chairman of a committee to reform state judicial procedures, they presented the
Massachusetts Practice Act of 1851. "It was considered a model of judicial reform and was approved by the legislature without amendment." At the time, Curtis was viewed as a rival to
Rufus Choate and was thought to be the preeminent leader of the
New England bar. Curtis came from a politically connected family, and had studied under
Joseph Story and
John Hooker Ashmun at Harvard Law School. His legal arguments were thought to be well-reasoned and persuasive. Curtis was a
Whig and in tune with their politics, and Whigs were in power. As a potential young appointee, he was thought to be the seed of a long and productive judicial career. He was appointed by the president, approved by the Senate, elevated to the Supreme Court bench, but was gone in six years. ==Supreme Court service==