The track debuted at number one in their native Britain and became the nation's sixth fastest-selling single of the year, shifting 145,000 copies in its first week of release in May, 2013.
It has since surpassed the one million sales milestone in the U.K., making it only the 148th single to do so since the singles chart began in November, 1952, according to experts at the Official Charts Company.
Smith has since gone on to forge a hugely-successful solo career for himself, with his first album, In The Lonely Hour, recently becoming the U.K.'s fastest-selling debut record of the year (14) so far.
He says, "The fact that La La La has sold so many copies and broken so many boundaries is unbelievable.
"I was actually really worried about releasing that track because I didn't want to be known as a featured artist. I couldn't have been more wrong!"
Other recent U.K. releases to hit one million sales include Pitbull's 2011 single Give Me Everything and Katy Perry's 2010 track Firework.