Mendes, the 19-year-old Canadian star, also won the best-song trophy for "There's Nothing Holdin' Me Back", to the delight of fans standing in a polite onstage mosh-pit at the city's SSE Arena.
"I'm so confused why is Rita Ora walking down the red carpet in a dressing gown and towel around her head?"
"I'm not really sure how I got this, 'cause I haven't had an album out in a few years" he quipped in his acceptance speech, adding "I got one coming though".
Russian-Backed TV Network's Production Company Registers as Foreign Agent
Moscow has repeatedly denied the allegations of election meddling and said it views the actions against RT as an unfriendly act. An RT television broadcast van is seen parked outside the Luzhniki stadium ahead of an global soccer match in Moscow.
Notable performances included local London grime star Stormzy, who took to the stage poking out of the sunroof of a police auto, and Travis Scott, who entered standing atop what looked like a floating hawk. "You are fantastic, thank you".
Jared Leto's band Thirty Seconds to Mars won best alternative act, and Leto used his time on stage to discuss immigration, saying: "We are Americans, a land of immigrants, and we just want to say we welcome you with open arms and open hearts and we love you".
U2, who played an MTV-sponsored concert in London's Trafalgar Square on Saturday, received the Global Icon award from Leto, who said "U2 changed my life". This is for you, this is about you and we stand here proudly on this stage tonight in celebration of you so thank you.
"Their songs are prayers, their concerts a church", he said.