The 37-year-old Rajiv Gandhi Khel Ratna awardee shot scores of 121 and 13 in the qualification and finals, respectively, to finish on the podium with the yellow metal dangling around his neck.
It was some achievement by the former world number one Indian and world champion as he was up against Australia's Diamond, who had won the top prize in the 1996 Atlanta and Sydney Olympics in 2009, besides winning five gold medals at the Commonwealth Games.
Manavjit scored 14 out of 15 targets in the semifinals. The Indian sharpshooter had last won at the World Cup stage four years ago.
The bronze medal went to Alexey Alipov of Russia who shot 121 and 12. Ireland's Derek Burnett, who topped the scoring in the qualifications with 123, failed to replicate his form in the semifinals where six shooters competed.
"We had some of the world's best shooters competing today. Winning the match against Michael Diamond has been definitely challenging but at the end you have to hit your own target, and the final score is what it is," Manavjit said after his win.