Mandic claimed Serbia's first Taekwondo Olympic gold when she beat Anne-Caroline Graffe of France 9-7 in the women's +67kg final.
Mandic, 21, was making her debut at the Games, but showed no nerves as she beat Beijing 2008 champion Maria del Rosario Espinoza and then European gold medallist Anastasia Baryshnikova en route to the final.
The Serbian continued her momentum against Graffe, landing a head kick in the second round to lead 5-4 going into the final two minutes.
And Mandic was not to be denied as she closed out a memorable victory.
The 2008 Olympic champion Espinoza came through the repechage to beat Cuban Glenhis Hernandez and claim bronze, along with Baryshnikova.
Meanwhile, Rome policeman Molfetta took gold on judges' decision after a classic final of the men's +80kg against Anthony Obame, whose silver was a first Olympic medal for Gabon.
Obame had scored an early head shot and looked to defend a 6-3 lead going into the final round.
However, despite landing another head shot, which was upheld on appeal from the Italian corner, Obame could not hold out.
With just 15 seconds left Molfetta, European champion in 2010, somehow found a head score of his own to send the contest into sudden death.
Neither man could make the breakthrough during the extra period, so it went to the judges' superiority decision - which was awarded to the Italian.
Earlier, wild card Daba Modibo Keita was unable to try to become Mali's first ever Olympic medallist when he could not take to the mat for the bronze match. Robelis Despaigne of Cuba was awarded the medal.
Liu Xiaobo of China claimed the other bronze when he edged a tight contest against Bahri Tanrikulu 3-2.