When the case came before the high court Adams, who was nearly deaf, was in rare form. The hearing took place in the Old Supreme Court. His actual argument lasted 8˝ hours, over two separate days. (One of the justices died, causing a delay.)
As he spoke for the Amistad captives, the ex-president pointed to the framed Declaration of Independence. With emotion he asked:
Is it possible to speak of this demand (to send the Africans back to Cuba) in language of decency and moderation? ...Has the expunging process of black lines passed upon these two Declarations of Independence in their gilded frames? Has the 4th of July ‘76 become a day of ignominy and reproach?
No other case, before or since, was precisely on point. Adams had little “legal” precedent. What he did have counted most. His main thrust was the eternal law of nature wherein every person is free. And it was that law - the eternal law - on which Adams staked his claim.
...I know of no other law that reaches the case of my clients, but the law of Nature and of Nature’s God on which our fathers placed our own national existence.
Associate Justice Joseph Story told his wife (in a letter) that Adams’ argument was "...extraordinary...for its power, for its bitter sarcasm, and its dealing with topics far beyond the records and points of discussion." In short, Adams gave the Supreme Court no choice. The justices had to do the right thing.
It took about one month. Justice Story wrote the majority opinion. There was one dissent.