Mandic upset top-seeded Anne-Caroline Graffe of France in a tight final by a score of 9 to 7.
Earlier in the day, Mandic knocked out two of the division favourites, defending Olympic champion Maria Espinoza of Mexico and second-ranked Anastasia Baryshnikova of Russia. Espinoza later won a bronze medal.
Mandic was frequently on the offensive in her fights on Saturday, often striking her opponents first with a targeted kick to the body before quickly retreating so they couldn't counter-attack.
Mandic, 20, won a bronze at the world championship last year.
The other bronze medal went to Russia's Anastasia Baryshnikova.