Soni clocked 2 minutes, 19.59 seconds to improve on her own mark from Wednesday's semifinals by 0.41 seconds.
It was the sixth world record in the pool at these games.
Satomi Suzuki of Japan touched in 2:20.72 to take the silver medal and Iuliia Efimova of Russia finished in 2:20.92 for bronze.
Toronto's Martha McCabe was fifth.
Soni was also favoured to win the 100 breast but took silver behind surprise 15-year-old winner Ruta Meilutyte of Lithuania.
Meilutyte did not enter the 200.