Even celebrities get a little starstruck every now and then!
The 19-year-old singer was recently in London, England, performing for Queen Elizabeth II's 92nd birthday.
"I stood next to the Queen for about 15 minutes, waiting to go on stage," Mendes told ET's Nischelle Turner. "We didn't technically meet, but I was standing next to her. It was really awesome! It was a lot more relaxed then I thought it would be, in the best way. That was just -- truly, I'm having a really great week."
"She's very funny," he continued."The whole royal family is very normal and just, like, super likeable and amazing."
Mendes said he also spotted Meghan Markle and Prince Harry, but didn't have the guts to talk to them!
"I didn't [meet them], I actually chickened out," he confessed. "I walked by the hallway and I saw them in there and I was like, 'Should I go say something? Nah.'"
"Usually I don't freak out like that, but I was really... I was surprised by how nervous I got in front of them," he added.
Although Mendes says he won't be at the actual royal wedding on May 19, he'll be watching "like everyone else."
One person who is expected to attend is Amal Clooney's go-to London hairstylist, Miguel Perez. A source told ET on Tuesday that the stylist is expected to style Markle's hair on her special day.
"Yes, I do Meghan's hair," Miguel reportedly told The Mail this week. "I do Amal's, and Meghan is friends with her. I also do George [Clooney]'s, I do the whole family. Amal put Meghan and me together when she moved here [to London]. I do her color, her cut and her styling."
"We have talked about it [the wedding] and we have a plan," he continued. "I can't say any more, but we know what we're doing."
Hear more in the video below, and be sure to keep up with all of ET's coverage of the royal wedding as we count down to the former Suits star's journey to the altar!