Lee Shi-un began his acting career with a bit role in the 2001 film Kick the Moon. In 2009, he joined the main cast of the MBC drama Friend, Our Legend. In 2012, he received a boost in his popularity after starring in the hit tvN series Answer Me 1997. In 2015, he joined the SBS legal thriller Remember—Son’s War, which scored him a Special Award nomination at the SBS Drama Awards. He was then nominated for Best New Actor at the 2016 MBC Drama Awards for W—Two Worlds. In 2017, he took home the Rookie Award at the MBC Entertainment Awards for his work in the popular variety show I Live Alone. That same year, he received acting nominations at the MBC and SBS Drama Awards for his performance in Two Cops and Reunited Worlds, respectively.

Lee Shi-un attended the Seoul Institute of the Arts. He and actress Seo Ji-seung tied the knot in December 2021 on Jeju Island.