He is best known for his roles in the television dramas All In (2003), Save the Last Dance for Me (2004), New Heart (2007), Protect the Boss (2011), Secret Love (2013), Kill Me, Heal Me (2015), Innocent Defendant (2017), Familiar Wife (2018), Doctor John (2019), The Devil Judge (2021) and the film My PS Partner (2012).

[2] Upon the suggestion of his then-manager, he began using the stage name Chae Ji-sung, then decided to drop the surname "Chae," and became known simply as "Ji Sung"[2] After several minor roles, he began playing bigger parts, notably in the television dramas Wonderful Days (2001) and Sunshine Hunting 2002),[4][5] as well as the North-South romantic comedy film Whistling Princess (2002).

[10] He then made a guest appearance playing Bae Jong-ok's college boyfriend in "Outing", a two-episode arc written by Lee Kyung-hee for the omnibus drama Beating Heart (2005),[11] followed by a supporting role in Kim Dae-seung's period thriller Blood Rain (2005).

[17] During a scene where his character cries after failing to save the life of a former comfort woman, Ji Sung said he thought of his maternal grandmother who'd recently died, remembering that she'd told him her only pleasure was watching him in reruns of All In.

Contrary to his cheeky, playful character, Ji Sung said that he's taciturn and shy in real life, but that the drama made him find a different side of himself.

[19] In 2009, Ji Sung collaborated again with writer Choi Wan-kyu and director Yoo Chul-yong of All In in the revenge drama Swallow the Sun, adapted from the novel by Kang Chul-hwa, and shot on location in Jeju Island, Las Vegas and South Africa.

Ji Sung trained hard to play the tough, strong-willed protagonist who goes to prison for his boss and later becomes involved in the city development of Jeju.

His fictional character becomes a "kingmaker" to Yi Seong-gye (played by Ji Jin-hee), the general who leads the overthrow of Goryeo and establishes the Joseon Dynasty, becoming its first king Taejo.

[35] Ji Sung next appeared in Confession, a 2014 neo-noir film that brutally explores the aftermath of three men's friendship after the death of one's mother.

[37][38][39][40] The series was a hit and developed a cult following, winning Ji Sung the "Daesang (Grand Prize)" at the MBC Drama Awards.

[49] In 2020, Ji appeared in tvN's travel show RUN alongside Kang Ki-young, Lee Tae-sun and Hwang Hee.

Ji Sung played the roles of the twin brothers who pair up to uncover the mystery of a murder that occurred 22 years ago.