[4] He was also active in Dongsung-dong since the second semester of his senior year, including guest directing play Bison, a performance for the drama class at Deoksung Women's University [ko], as a part-time job.

He met Kim Min-ki and was cast in hit Korean adaptation of the German rock musical Subway Line 1 [ko].

Sul participated in this work from the premiere in 1994 to 1996, playing all but two of the 80 roles, accumulating various experiences and acting skills and earned big success.

In 1996, Sul made his screen debut in his first film A Petal, playing the role of Woo-ri, a college student who is chasing the whereabouts of the female lead girl (Lee Jung-hyun) at the recommendation of director Shim Kwang-jin, a college classmate who was taking directing lessons from director Jang Seon-woo.

In 1997, Sul met Cha Seung-jae, the CEO of Sidus FNH, the producer of the film Girls' Night Out, which released in 1998.

He then signed management contracts with Sidus HQ and made his breakthrough, with major roles in Rainbow Trout, Phantom: The Submarine, and The Bird That Stops in the Air (1999).

[10] In early 1999, Sul was selected for the lead role through an audition in director Lee Chang-dong's film Peppermint Candy.

In addition, director Lee Chang-dong revealed the reason for casting Sul Kyung-gu as follows.Unlike other actors, I rather liked that he hesitated and said that he had no confidence.

He looked weak in charisma with an ordinary mask, but his face was different every time I saw him, and that seemed to enable him to express various colors as well as good and evil, so he was cast.

[12]In film Peppermint Candy, Sul played Kim Yeong-ho, a suicidal man devastated by the two-decades of historical change his country undergoes.

Sul next appeared in a romantic comedy I Wish I Had a Wife with Jeon Do-yeon in 2001, played the role of Bong-su, an ordinary old bachelor bank clerk who yearns for romance.

In November, he acted together with Cha Seung-won in a box office hit Jail Breakers by director Kim Sang-jin.

[14] In 2003, Sul starred in Silmido directed by Cinema Service founder Kang Woo-suk, which became the first Korean film in history to gross 10 million admissions.

[15][16] His next role was as the title character in Rikidōzan, about the legendary ethnic Korean pro wrestler who became a national hero in Japan in the 1950s.

In October 2005, Sul Kyung-gu played the role of the male protagonist Andy in A. R. Gurney's Love Letters by the theatre company Hanyang Repertory.

[28] In 2013, Sul appeared in three movies Cold Eyes,[29][30] The Spy: Undercover Operation,[31] and Hope and mobilised a total of 1681 million viewers in 2013.

[39] Sul began teaching acting in 2014, at his alma mater Hanyang University, as an adjunct professor in the College of Performing Arts.

Sul played the role of Jang Nam-bok, a farmer who is suddenly dragged to the battlefield with a new-born baby in the war film Western Front, the directorial debut of playwright Cheon Seong-il.

[56] The film, inspired by the Sinking of MV Sewol tragedy, deals with the struggles faced by a couple who lose their son in a tragic accident.

[62] Sul and his wife Song Yoon-ah supported the entire cost of surgery for Ha-jin, who was suffering from congenital heart disease.

[63][64] In April 2014, Sul Kyung-gu, along with Song Yoon-ah, said through the Korean Committee for UNICEF, "I felt pain that is difficult to express in words while watching the Sewol ferry disaster.