Now, at 21, McCartney is out to show everyone that he's versatile, and his career shift is going just as he planned. It's reminiscent of the transition
"The comparison alone is pretty cool," he told MTV News. "Like Justin, as I'm getting older, I'm learning more and more about music and learning where I fit as far as style. ... Growing up, I listened to R&B music. I listened to a lot of Michael [Jackson] and Aretha [Franklin] and [Ray] Charles. As you get older, it does come across in your music."
And with this shift in musical style on his album Departure, released earlier this year, came some criticism from fans, who weren't used to McCartney putting out more mature music. "My fans were very surprised. There was a lot of shock value, especially with ... how different it is vocally," he said. "The last thing they saw before that was 'Beautiful Soul,' and my hair was down to my a--. ... I think it took awhile for them to jump onboard.
"There were definitely some mixed reviews, to put it nicely," he added. "I think the one thing that came across was the honesty. As soon as you're dishonest, [the fans are] the first people to tell you. It was an organic transition for me, given that this is what I was comfortable singing."
He said he has seen his audience grow from predominantly female to including some guys. "I see a lot of my audience is mainly girls, but I see a lot of their boyfriends now coming. That never would have happened four years ago. They're curious."
But McCartney isn't just busy making music for himself. He co-wrote Leona Lewis' smash debut single, "Bleeding Love," and he's currently in the studio with Toni Braxton and hopes to work with T-Pain, whose song "Buy You a Drank" he recently covered.
"We always had this sort of idea of what it would sound like slow," he said. "It just sort of worked. It felt cool. I ran into T-Pain at the AMAs. I don't think he expected me to cover a T-Pain song."
He also said he'd love to get in the studio with artists like Rihanna, Chris Brown and Michael Jackson in the near future. "Whenever I have time, I try to get in the studio and write, whether it's for me or other artists or my catalog of music. It's definitely one of my favorite parts of the music industry. I know a lot of people who are writing and producing Michael Jackson's new album. That would be a dream come true. ... There's talk of maybe working with Ludacris. We'll see."
On top of lining up some work in the music industry, McCartney also hopes to line up more acting jobs as well. He recently made a cameo on "Law & Order: SVU" and will also appear on "Greek." "I would love to work with [Martin] Scorsese. He's one of my favorite directors of all time. He's definitely got his thing that I'd love to be a part of."
He even has an idea of how he'd like to be cast. "Maybe some Irish Catholic drug addict," he suggested. "Scorsese is known for his weird, offbeat, dramatic films."
Regardless of what his next project is, McCartney has one goal: to keep people guessing. "I want to keep everyone curious," he said. "I'm always going to throw curveballs out there."