Sung Joon began his entertainment career as a model, but soon switched to acting and debuted from the drama White Christmas in 2011. The same year he was cast in a supporting role in Lie to Me. In 2012, he was cast in a leading role in the teen drama Shut Up Flower Boy Band, where he played a free-spirited leader of a rock band.
Sung Joon maintains close friendship with fellow models-turned-actors: Kim Young-kwang, Lee Soo-hyuk, Kim Woo-bin and Hong Jong-hyun – whom he has worked onscreen in White Christmas. They were given the nickname of Model Avengers by their fans.
In 2014, he was cast in the third installment of TvN's romantic comedy I Need Romance 3 which he portraits a character of a care-free Songwriter. Later that year he converts into a plastic surgeon for the drama Discovery of Love.
In 2015, Sung was cast as the lead in mystery melodrama High Society, playing a man who succeeds in spite of his poor family background. The same year, he had a supporting role in Hyde Jekyll, Me, a romantic comedy where he played a hypnosis specialist.
In 2016, Sung played the male lead in the romantic comedy drama Madame Antoine: The Love Therapist alongside Han Ye-seul.
In 2017, Sung starred in the mystery comedy, Ms. Perfect alongside Ko So-young, playing a Lawyer.