Lee Won-geun made his acting debut in 2012 with a supporting role in the hit MBC sageuk The Moon That Embraces the Sun, which was named Best Drama at the 48th Baeksang Arts Awards. In 2015, he became part of the main cast of the KBS2 teen series Sassy Go Go, in which he was nominated for Best New Actor at the KBS Drama Awards. In 2016, he starred in the movie The Net, which won him Best Actor at the Directors’ Week competition of the Oporto International Film Festival. His work in the movie also earned him a Best New Actor nomination at the 37th Blue Dragon Film Awards. That same year, he acted alongside Kim Haneul and Yoo In-young in the psychological thriller film Misbehavior. He bagged Best New Actor nominations at the 22nd Chunsa Film Art Awards and the inaugural The Seoul Awards for his performance in the movie.

Lee Won-geun enlisted for his mandatory military service on June 13, 2019 and was discharged on January 27, 2021.