Ryu began his acting career in theater, subsequently becoming one of the most versatile supporting actors in Korean film and television.

During his time there, he studied alongside classmates such as Ahn Jae-wook, Jung Jae-young, Lee Hae-young, Hwang Jung-min, and Im Won-hee.

His classmates Hwang Jung-min, Jung Jae-young, Im Won-hee, and Jang Jin, who was one year his senior in college, also took part in the project.

[9] Ryu worked various part-time jobs to make a living and gain life experiences, including delivering pig's feet, interior decorating, carrying cabbage at Garak Market, and car washing.

He and Yoo Hae-jin shaved their head to perform the avant-garde play Duta (The Road to Gudo) by Professor Kang Man-hong at the La MaMa Experimental Theatre Club in New York.

[14][15][16] After participating in the Edinburgh Festival Fringe in 1999, Nanta toured over 20 countries around the world, including Europe, Japan, Southeast Asia, Australia, and North America, and has recorded an audience occupancy rate of over 90% to date.

[16] In 2002, they made their debut in New York, USA, with a performance on Broadway and an appearance on NBC's Today Show in 2002, predating Psy.

Prior, due to Nanta, Ryu declined a script from Director Jang Jin for a play called Heotang.

In this film, he appeared as a slow-speaking farmer from Jeolla province with a sagging belly, delivering a reliable performance as a father who, though disdainful of his son's first love, still embraced him.

[4] He then appeared as a rival as prosecutor Seong-jun, a colleague of Choi Yoon-gi (Cha Seung-won) in Jang Jin's film Murder, Take One [ko] (2005).

In this film, he played a contractual torture technician, portraying a middle-aged worker permeated with fatigue, unable to receive proper overtime pay despite working overnight shifts.

[18]Ryu then appeared in a series of director Jang's films, including 'Good Morning President' (2009), and 'Quiz Show' (2010), making a solid impression on Chungmuro.

7,[20][21] the royal adviser Heo Gyun in Masquerade, an officer of the North Korean People's Army in The Front Line, a hard-nosed general of the Qing Dynasty in War of the Arrows, a ladies' man in All About My Wife, a gambling husband who fears his wife in The Quiz Show Scandal, a refined gay man in Personal Taste, a vengeful gangster in Secret, a reporter on the trail of a story in The Recipe, and the titular figure in The Piper.

Based on Kang's webtoon of the same name, the series is a supernatural drama that follows three high school students and their parents as they discover their superpowers.

[30] In the series, Ryu plays the role of Jang Ju-won, Hui-soo's father who runs a fried chicken shop.

After seven days of availability, it became the most watched Korean original series on Disney+ globally and Hulu in the United States, based on hours streamed.

[33] Following the success of "All About My Wife," Ryu Seung-ryong became a sought-after endorser for brands like Samsung Life Insurance, Kumon Learning Center, and SK Telecom.

Samsung Life Insurance noted an increase in advertising interest after the movie reached 10 million views.

SK Telecom experienced a surge in sales after airing the commercial, making it the top-selling new product in March.

[34] Following the success of 2014 historical film The Admiral: Roaring Currents, Ryu Seung-ryong starred in The Baedal Minjok advertisement with the slogan 'What kind of nation are we'.

His commercials ranged from cinematic broadcast ads to clever outdoor campaigns, strategically placed in relevant locations.

[37] In April 2024, Home Chicken SASE (CEO Kim Gwang-seon) announced that it has selected actor Ryu Seung-ryong as its new brand model and was launching a campaign.

[38] In 2015, Ryu donated 10 million won to support the independent film The Claws of the Five Generals, an adaptation of an award-winning play.