Hysong and three other competitors ended with the same height of 5.90 metres.
Hysong won the gold on countback after he failed less heights earlier in the competition. His compatriot Lawrence Johnson took the silver and world champion Maxim Tarasov of Russia the bronze.
The American outsider's previous best height was 5.85, while his previous best finish came in the 1999 world championships when he was fourth equal.
German Michael Stolle was the unlucky 5.90-metre man to miss out on a medal.
With six-times world champion Sergei Bubka of the Ukraine failing to qualify, the final was always going to be wide open.
Tarasov, Australian Dimitry Markov, South African Okkert Brits and Germany's Tim Lobinger were the only men in a final field of 12 to have cleared 6.00 metres.
Lobinger, however, was the first to fall by the wayside when he failed at 5.70.
Brits was the next to go when he failed three times at 5.90 while Markov, the former Belarussian who represented Australia for the first time last year, missed 5.90 twice, then upped the bar to 5.96 only to fail again.
Hysong had been vaulting smoothly and flew into uncharted heights with his first attempt at 5.90. Stolle and Tarasov needed three to get over while Johnson managed it at the second attempt.
All four looked capable of clearing 5.96 but failed, leaving the U.S. to celebrate a double success and a return to the top of an event they used to dominate.
Americans won every pole vault gold from 1896 to 1968 -- the longest winning streak by one nation in any Olympic event -- but Hysong's success was the first since.
Jean Galfione, the 1996 gold medallist, failed to qualify, like Bubka, for the final. The Frenchman had been hampered by a recent lung operation.
Pole Vault
