Sprague was also a collector of historical documents and pamphlets and became the first person ever to gather a complete set of the autographs of the signers of the United States Declaration of Independence. He completed this task by February, 1833, according to correspondence with friend Jared Sparks at about that time. He also gathered a collection of the signatures of all of the members of the Convention which framed the Constitution of the United States in 1787, and a complete set of the autographs of the Presidents of the United States and all the officers of the United States government during the administrations of Presidents Washington, John Adams, Jefferson, Madison, Monroe and John Quincy Adams. This latter collection included signatures of the Presidents, Vice Presidents, all the members of the President’s Cabinet, and all of the Justices of the United States Supreme Court and all of the foreign ministers. Further, he collected the signatures of all the military officers involved in the American revolutionary war, from all nations, during the whole war. He collected signatures of great men of the Reformation and great skeptics. He even owned a copy of the autograph by Saint Augustine. He was America's foremost philographer by the time of his death. His autographs, numbering nearly 100,000, probably the largest private collection in the world at that time, were left to his son.