Mendes performed "There's Nothing Holdin' Me Back", the victor of "Best Song".
The Irish band dedicated the award to label boss Chris Blackwell of Island records, who signed them nearly 40 years ago.
French DJ David Guetta, who performed in front of the youthful crowd, won for best electronic act. Despite receiving six nominations - the most of any artist - Taylor Swift failed to win any awards.
The majority of the winners were picked by music fans in an online vote, from 12 categories listed and a further 32 separate prizes for different countries and regions.
Other winners at the event, which celebrates the worldwide music scene, included Coldplay (for best rock act), Dua Lipa (for best new act) and Zayn (for best look). It was a busy night for Jared as he also presented U2 with the Global Icon Award at the spectacle, and thanked the Irish rockers for changing his life.
"U2's impact on music, pop culture and social issues around the world has been tremendous", said Bruce Gillmer, producer of the awards ceremony.
"I'm so confused why is Rita Ora walking down the red carpet in a dressing gown and towel around her head?"