Yamechi lifted 100kg to take the snatch and then managed a personal best of 130kg in the clean-and-jerk to win the overall title as well.
After both events, the Cameroonian's coaches carried her off the stage in delight.
India's Neelam Sethi Laxmi took silver in all three categories with Nauru's Sheba Deireragea finishing with three bronze medals.
England's Sharon Oakley broke the British record when she lifted 102.5kg in the clean-and-jerk but only finished fifth overall.
There was no medal joy either for veteran English lifter Juliana Auguste.
The 37-year-old was sixth overall.