He also won four Mobo awards, co-wrote an album with Mary J Blige and scooped an astonishing six Grammy nominations.
The 22-year-old, from Cambridgeshire, is the son of a London banker who was fired for spending too much time trying to jumpstart her offspring's pop career.
He got his break singing on other people's songs - Latch, by dance act Disclosure, and La La La by soul producer Naughty Boy - before scoring his own record deal.
Much has been made of his soaring, multi-octave vocals but his technical abilities never mask his emotions.
In the Lonely Hour was love letter to a man who never returned Smith's affections, and its honesty spoke to record-buyers in their millions.
Ahead of the announcement of the Sound Of 2015 next week, the singer reflects on the highs and lows of his first year in the spotlight.
I had hopes of number ones, but I never thought it would actually happen.
When you come from being a featured artist there's a sudden fear: "Oh God, was it me that people liked - or was it Disclosure and Naughty Boy?"
So when Money On My Mind came out, it was a reassurance - a boost of confidence for me. Just letting me know "things are good, people like your songwriting".
That will never happen again - going to the Brit Awards knowing you already have a Brit Award. It was the first time I'd ever been, and I was number one that week. That was very, very surreal for me. I really celebrated hard that night.
SNL was the scariest moment of my life, ever. It was the first time I played Stay With Me or Lay Me Down on television.
I don't think I'll ever be that scared again...  Four million people watching and no-one knew who the hell I was. Four days previous to the SNL performance, I remember reading tweets from people saying "who the f is Sam Smith?" [laughs]
But it changed my life. You could feel it the morning after. I walked out of my hotel and people recognised me. Stay With Me went to number two on iTunes. It was mental.
Sam's album shifted 166,000 copies in its first week on sale in America, the most ever for a debut by a British male artist. It was only kept off the top by Lana Del Rey's Ultraviolence.
Every UK artist prays for US success but, genuinely, it was 50/50 for me. It was either going to work or it wasn't. What I didn't expect was for it to work hand-in-hand with the UK. I didn't think I'd be going to America untilâ€¦ well, about now, actually.
Was I cursing Lana Del Rey? I wasn't, actually, because it gave me something to work towards. If I'm going to get number ones all the time, it leaves nothing for me to achieve in the future.
Also, what I've realised now is that it isn't about chart positions. I was number two for the first week, and I was number two for the second week. It's much more important to keep selling and have an album that doesn't actually leave the charts.
I was on the same time as Dolly Parton, so I was freaking out the whole day. She had the biggest audience in Glastonbury history and I was up against her. Surprisingly, my audience was massive, which was incredible. But that was a tough one.
I'm a massive fan of pop culture - but there was a lot of twerking that night. A lot of explosions. Even for me, sometimes, I was a bit like "woah, woah, woah!" I don't like being the only person standing there and singing. But it worked to my favour that night.
Meeting Beyonce was really weird. I thought I'd freak out. I'd put her on such a high pedestal she'd become un-human. Then you meet her and she is just a human. A very talented human, but just a human.
I was the only person who was allowed to watch her rehearsal, so I said, "I loved your sound-check. I think it's one of your best performances yet." And she said, "thank you. Your voice is like butter."
I keep telling people she said chocolate, but she actually said butter.
I never, never expected that - but it all happened organically, which is just how I like things to work.
Mary J Blige, Elton John, Chaka Khan - those three have been incredible to me. It's something I've learned. When there are up and coming artists experiencing fame for the first time, it's important to make sure they're OK - because it's quite an ordeal, going through it all. And they were amazing.
Mary J Blige is incredible. And still, to this day, will randomly text me making sure I'm ok. Her best advice was "don't be afraid to be blue" - so don't be afraid to write sad songs.
What's happening now is incredible, but no matter how many awards I may get, I will be tapping myself on the head at the end of this project and reminding myself "this is my first album". You can't truly say that you are imprinted in music, in society, unless you have three or four albums out.  I want a long career. That has been my aim from day one.
Sam Smith's album, In the Lonely Hour, is out now.
Sam Smith topped the BBC's Sound Of 2014 list last January, and went on to sell a million copies of his debut album, In the Lonely Hour, on both sides of the Atlantic.