The Baron of Arizona
In the late 19th century, due to various treaties the US was bound to honour historical Spanish and Mexican land grants in what was now US territory (Arizona and New Mexico in the present day). Show that King of Spain had granted this land to your forebears - it was yours.
In a scam that took more than two decades to gestate and play out, Reavis bought a mine claim that also included the "Peralta grant", a deed and sheaf of papers that been bought from a poor Mexican miner. The deed granted rights to a large patch of land, albeit one that was poorly defined, and that in time was interpreted as encompassing more than 10,0000 square miles, swallowing critical towns, mines and railroads.
Travelling to Mexico, Reavis researched the authenticity of the claim, returning with an intricately documented history of how the land had been granted by King Philip V of Spain to a royal emissary in 1748. Many independent sources testified as to the existence and legitimacy of the grant: wills, letters from the king, proclamations, statements by priests.
Reavis wasted no time in pressing his claim. Railroads paid him for right-of-access, mines paid for quitclaims, even residents in the claimed area were harangued for trespassing. When his claim was challenged, Reavis was always willing to let his paperwork be examined. Surveyors and land commissioners seemed poised to confirm his claim, while numerous politicians and celebrities spoke out for him.
Further cementing his claim, Reavis found and married a descendant of the original claim-owner, and embarked for Spain to look for more proof. There they eventually found various other confirmatory documents about the claim and family history. Reavis returned to the US, styling himself "James Addison Peralta-Reavis". He formed a number of corporations for developing the land, and maintained houses in a San Francisco, St. Louis, and New York City, where they cultivated the sympathy of the rich and powerful.
Of course, it fell apart. In the room-full of documents that had been accumulated by this point, some were written with the wrong sort of pen while others had grievous spelling mistakes. Document archives that should had held evidence of the claim, did not. Papers of the same age didn't match.
It took years to resolve, but Reavis had spent years building up his con. He studied old land grants. He inserted bogus documents in archives in Mexico, Portugal and Spain. He criss-crossed claim territory, leaving evidence. He altered books. He slipped fake birth details into parish records. He bribed archivists. He even forged a stone monument in a remote area of the desert.
Reavis was brought to trial in 1895 and was sentenced to six years in the Santa Fe penitentiary.