"It's the coolest fluke ever," says the 18-year-old about her meteoric rise - which is all thanks to one track.
That song is Here, a shy and sultry R&B jam about not belonging, which finds the singer trapped among awful people at an awful house party.
Built around the same slinky Isaac Hayes sample that powered Portishead's Glory Box, it racked up 50,000 plays on Soundcloud the day it was uploaded, and has since garnered nine million views on YouTube.
"I knew there was something special about that song but I didn't expect people to love it so much," Cara tells the BBC.
Born Alessia Caracciolo, the singer grew up on the outskirts of Toronto in an Italian home. Like her fellow Ontarians Drake and The Weeknd, her music is a laconic, millennial take on R&B.
"Canada is a really big melting pot of cultures, so we ended up with a giant mosaic of different music," she says, "but there's now a defined Toronto sound, this very dark, underground mixture of hip-hop and R&B."
As Here is released in the UK, Cara explains how the track came about, and what we can expect from her next.
Hi Alessia, how are you enjoying London?
Hey there! I'm good. I love it here. I don't want to be cliched but Buckingham Palace is beautiful and the old red telephone booths are really interesting to me. I've always wanted to see those.
Congratulations on the success of Here. How did the song come about?
It was based on a real party that I had gone to two years ago in my friend's basement.
I'd been to other parties before which were fine and I thought this would be the same - but it absolutely was not. When I got there, it seemed like everyone knew each other except for me. Everyone was sweaty and dancing and listening to music I'd never heard before. People were getting drunk and smoking and they were passed out on the floor. I felt way too uncomfortable. I just didn't belong.
Through the eyes of someone else, that might have been an amazing party - but it was not my thing. So I asked my mom to come get me and I went home early.
In the song you describe yourself as an "antisocial pessimist". Is that an accurate portrait?
I don't think that's true! Even in the song I say, 'Excuse me if I seem like an anti-social pessimist.' So, to someone watching me from the outside, I might have seemed that way. I was just annoyed with everyone, I didn't want to talk to anyone. But I'm not anti-social.
Which is good news, because you're in the wrong industry for someone who doesn't like big parties.
Or so you'd think! But I haven't been invited to any parties so far. I think everyone hates me now.
Maybe they're too scared to ask in case you write a scathing song about their soiree.
Yeah, they probably think I'm going to ruin everything!
The Isaac Hayes sample is so strongly associated with Portishead that using it in a new song is either incredibly brave or extremely foolhardy. What made you go with it?
It was actually the last thing we added. Two years after I wrote the song, it was still just a demo and then I went into the studio with these producers named Pop and Oak in LA. I played them the track and they said, 'We want to to work on it.'
Pop had the idea to pull up that song. He took four seconds of it and started putting it on a loop - then I started singing over that loop. And I realised, 'Wow, this really fits.' I don't know how he thought of putting that song in it, but I guess that's why he's a producer and I'm not!
You grew up in Brampton, a small suburb of Toronto. Was is a musical household?
It wasn't a major thing. Ninety per cent of my family are hairdressers and the other 10% are construction workers.
Music was something I found on my own. I got my first guitar when I was around 10 and it just all developed over time.
What's the first thing you remember listening to?
Other than the Sesame Street soundtrack, which I was obsessed with, the first artist I really felt I'd discovered on my own was Amy Winehouse. She was the first female artist I wanted to write like and sing like and be like.
I also loved the Black Eyed Peas, which seems really strange now, and as I got older, I got into Frank Sinatra and Michael Buble and Lauryn Hill.
So you were studying singersâ€¦
Oh yeah. Even Jessie J. I started getting into her music pretty early on, too. I'm a fan of real singers. I just gravitate towards that.
What was the moment you learned to sing as Alessia Cara instead of imitating other people?
The thing is that I always kind of did that. At the time, the big artists around were the pop divas. Everyone wanted to sing like Whitney Houston and Mariah Carey - these big voices who'd sing high, crazy notes. When I tried to do that, it just wouldn't work. So I had to do what I was doing, because I'm not that kind of singer.
Like a lot of aspiring musicians, you started posting cover versions on YouTube. Did you agonise over pressing the "publish" button the first time?
Yes, of course, it was a big step for me. I was so shy as a kid. But that was supposed to be my way of getting my voice out there without having to sing in front of an audience. I wasn't confident in myself and I wasn't sure I could sing. But the positive reaction from my friends and family at first, and then random strangers who were finding my videos, that gave me a lot more confidence.
Your brother filmed a lot of those early videos... You must have trusted him a lot.
He was the only person I would actually sing in front of. I would never, ever sing for my parents. I guess because he was younger at the time and I thought, "He probably can't even tell what I'm doing.'
He even falls over at the end of one of the songs.
That was real! It was so funny. I don't know if he did it on purpose to make me laugh but he was walking and literally collapsed to the floor. I was laughing so hard and so bad.
You didn't play your first proper gig until last year - and that was in Harlem. How nervous were you while you set up your amp and guitar?
I was just like, 'Oh gosh, I hope that they listen!' Because everyone was talking, and they weren't paying attention. But by the end of the second song people were all facing towards me. It was awesome. Some of them even gave me tips afterwards!
And now you're preparing to release your first album. Can we expect more songs along the lines of Here?
The songs I've been playing live - Wild Things, Scars, Too Beautiful and Here - are all in a similar vein conceptually. But sonically I go to different places.
And why is it called Know It All?
It's a sarcastic title. The album is so opinionated about the world - but at the end of the day I really don't understand what I'm doing at all.
Here is available now on Alessia Cara's Four Pink Walls EP.
She only played her first concert a year ago, but Alessia Cara has already been signed to legendary record label Def Jam, toured the world and sung live on national TV.