He was called upon by the New York City police to consult in a number of notable cases, including the hunt for the Zodiac Killer.
The British historian who was claiming them to be authentic eventually changed his mind and sided with Hamilton; the forger was unmasked and convicted in 1983.
He once owned a note that Queen Isabella wrote regarding the jewels she sold to pay for Christopher Columbus' discovery of the New World.
He owned the first draft of the Bill of Rights, and a letter written by Jesse James but signed with his pseudonym, Thomas Howard.
He owned another letter penned by Queen Victoria with advice for an unwed mother: "Let her wear a ring and no one need be the wiser".
Gary Taylor has said that Hamilton is a considerable paleographer, but some of his views on Shakespeare are controversial and "too recent to have been subjected to thorough scholarly scrutiny.
"[13] In 1983, Hamilton certified as genuine a letter purportedly by Joseph Smith that was forged by Mark Hofmann and sold to Gordon B. Hinckley, a member of the First Presidency of the Church of Jesus Christ of Latter-day Saints.