Since then, they've become a global force to be reckoned with, playing a string of sell-out shows in Australia and becoming the first British band to top the US Billboard charts with their debut album - which has since sold in excess of two million copies.
With their Up All Night: The Live Tour DVD out next week, Digital Spy caught up with Niall Horan to find out how the band are coping with international fame, what he makes of his dad's recent comments about his sex life, and what we can expect from the group's second studio album.
A lot has changed for One Direction in the last few months. How are you and the rest of the band finding it?
"Everything is good out here thanks! We actually managed to get in a little break after our Australian tour. I went home to Ireland for a bit of chillin'. We're now back out in America to start our tour today. It's the same show as the one we did in the UK - we're excited to show it to our American audience!"
"It's pretty exciting as things have been going better than we could have imagined over here. It's nice to be able to give something back to them after they've already given us so much. The venues are a bit bigger out here - I think tonight's venue holds about 9,000 people. The smallest show is 6,000 and the biggest is 20,000."
You seem to be playing a show almost every night; do you still get nervous?
"It does feel like that! When we first started out the thought of getting up on stage freaked us all out. The kind of crowds we get are very, very loud which helps. The bigger the crowd the better really! The noise calms your nerves."
Will your guitar be making an appearance out there as well?
"Yes, it will be making an appearance on stage over here as well! Hopefully on next year's tour we can bring it in a little bit more."
Watch One Direction perform 'More Than This' from the Up All Night: The Live Tour DVD below:
There's a review of one of your Aussie shows by a fan who got a little too excited... if you catch our drift.
"Yes I saw that! We see that sort of thing quite a lot. You can see girls getting too excited and they start crying or worse. In a lot of these countries they don't get a lot of gigs to go to, so when they get to see their favourite artist they take full advantage of it. Obviously the excitement builds up too much for some on the night and they get a bit...crazy."
You must have seen a lot.
"Sometimes I've seen some stuff that's a bit too much! Also, sometimes they'll queue outside a venue for ages for us in the freezing cold. Queuing tips for fans: wrap up and bring food!"
Your dad was recently talking to the papers about your personal life. Was that embarrassing?
"Erm... [pauses] not really I suppose. I don't get to see him much these days, but I've got my private life and he's got his. I try to keep him away from those kinds of things."
So you're not p*ssed off with him then?
"We're always talking to each other - we're close."
You've been recording your second album; how's it sounding so far?
"It's going really well, thanks! We've got some great songs that we're very excited to bring out and can't wait for the fans to hear. We're hoping to get it out towards the end of the year."
Are you writing songs for it again?
"We're writing as much as we can for this one. There's one song in particular that we're really happy with and we're recording it very soon. We've got a bit of time to get the album done so we're putting as much into it as possible. We're trying to keep the same sort of sound as before but create even better songs than the last one. There's always pressure on the second album - this one has to be the big one."
You worked with X Factor's Savan Kotecha and Ed Sheeran on your debut; who are you working with this time?
"Tom from McFly has just written a song for us and it's very, very good! He wrote a song on the last album too and we love his style. We've got another writing session with him soon which should be fun. We're also working with Ed Sheeran again - when we get back from America it's straight to the studio with him!"
Will there be any collaborations on this record?
"There aren't any we've discussed yet. We'd love to do a collaboration but you've got to find the right person to do it with. It would be weird if we did something with Eminem. You have to think outside the box! Can you imagine One Direction and Eminem?! That would be hilarious."
The Wanted have suggested forming a supergroup with you for charity. Would you be up for that?
"It would be just like Blazin' Squad! See you at the crossroads, crossroads, crossroads! That was a tune. It would be funny to see that, but we'll see what happens. I haven't heard anything about that yet."
Have you been starstruck by anyone in America yet?
"I was starstruck by Michelle Obama. She's an amazing-looking lady, and I'm a massive Barack Obama fan anyway. Katy Perry still gets me every time. She's very funny in person! We met at the Teen Choice Awards and she pulled my cheeks apart and told me how cute I was. My life was literally flashing before my eyes!"
What does the next year hold for One Direction?
"Our tour next year seems to be turning into a bit of a world tour! It's almost sold out everywhere already so I guess we'll be spending most of next year doing that. We miss home a lot, but we love being on the road. Hopefully people will stick with us for the second album - we don't want to keep our fans waiting too long. It's tough, but it's nice to know this time that there are so many people behind us."
One Direction's Up All Night: The Live Tour DVD is out next week. You can pre-order the DVD here.
Watch behind-the-scenes footage from the DVD below: