"Holding Me Back" had already won the EMA for best song earlier in the evening, with Mendes thanking his "best friends, Teddy [Geiger], Scott [Harris] & Geoff [Warburton], for writing this song with me".
Also making an impact on today's Official Chart Update at Number 4, and with potential to make big gains this week, is Eminem's brand new single Walk On Water feat. The award ceremony is taking place at the SSE Arena Wembley in London.
Coldplay was named best rock act, while the best-alternative prize went to Thirty Seconds to Mars.
Taylor Swift, who was expected to be one of the big winners of the night with six nominations, failed to win any awards.
Jack Swagger signs with Bellator MMA
Swagger's catchphrase "We the People!" raised his profile, but earlier this year, he asked for his release from the company. The 35-year-old Hager has spent the last 11 years under the WWE and its affiliated developmental territories.
The minor blip didn't affect the rest of the night for Stormzy though, after he picked up the award for Best UK and Ireland Act at the ceremony.
Irish rockers U2 were presented with a "Global Icon" award, in recognition of their decades at the top of the music industry.
British R&B singer Rita Ora performed the hosting duties, as well as a mashup of her 2017 hit "Your Song", alongside her new single "Anywhere".