Documents bearing Fossett's name, including a pilot's licence, money and clothing, were found by Preston Morrow, on Monday.
The Federal Aviation Administration, which would have issued the discovered documents, said it was trying to confirm their authenticity.
After an extensive search failed to find any trace of the millionaire businessman or his plane he was legally declared dead by a court in Chicago in February.
Mammoth Lakes, on the eastern side of the Sierra Nevada range, is about 160km from where Fossett began his solo flight, a flight that had been expected to last for three hours.
The identification cards discovered by the hill-walker provided the first possible clue to Fossett's whereabouts since he disappeared on September 3, 2007, after taking off from a Nevada ranch owned by Barron Hilton, a hotel magnate.
In 2002, Fossett became the first person to circle the globe solo in a balloon and had about 100 other world records to his name.