Mendes, attending his first Met Gala on Monday, was wearing a maroon Tommy Hilfiger suit with gelled hair while Baldwin donned a grey, off-the-shoulder Tommy Hilfiger dress with a flower crown and pink hair.
Mendes was seen on the red carpet and around the Metropolitan Museum of Modern Art fixing Baldwin's long dress and being extra careful not to step on it.
The pair's Met Gala apperance comes after Mendes fueled dating rumors when he posted a photo of himself striking a pose next to Baldwin on Instagram in April.
Baldwin, 21, and Mendes, 19, have been linked together since the fall. Baldwin, a model and daughter of actor Stephen Baldwin, said at the iHeartRadio Music Awards in March, however, that she wasn't dating anyone.
Mendes recently announced that his his upcoming, self-titled third studio album will arrive on May 25.