Benjamin Curtis was born November 4, 1809 in Watertown, Massachusetts, the son of Lois Ribbins and Benjamin Curtis, the captain of a merchant vessel. Young Curtis attended common school in Newton and beginning in 1825 Harvard College, where he won an essay writing contest in his junior year. He graduated in 1829, a member of Phi Beta Kappa. He subsequently graduated from Harvard Law School in 1831 and was admitted to the bar the following year.
In 1834, he moved to Boston where he joined the law firm of Charles P Curtis Esquire.
In 1836, Curtis participated in the Massachusetts "freedom suit" of Commonwealth v. Aves on behalf of the defendant. 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 in Boston, Slater fell ill and 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' 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. Curtis was one of the attorneys who argued the case on behalf of Aves.
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. While previous decisions ruled that slaves voluntarily brought into a free state, and who resided there many years, became free, Aves was the first decision which held that a slave voluntarily brought into a free state became free from the first moment of arrival. The decision in this freedom suit engendered controversy and helped to alienate the South. It was in contrast to the Dred Scott decision, in which Curtis participated as an Associate Justice of the Supreme Court.
Curtis became a Harvard Fellow 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."