The 25-year-old former Hearts wingers had originally signed a new six-month deal with The Lions during the summer, despite being offered longer deals elsewhere, and has impressed manager Kenny Jackett with his performances this season to earn himself the extension he was so desperately seeking.
"I'm delighted to get it all sorted. Now I'm looking to help the team get promotion," he told the club's official website. "Millwall is a club going places - there's a good feeling about the place, and that's why I signed.
"I want to be part of a successful team. I'd like to thank the manager, my team-mates and the supporters - everyone's been great to me, especially since my knee injury."