Im Si-wan was born on December 1, 1988, in Busan, South Korea. He attended Busan Gudeok High School, Busan National University, University of East Broadcasting Arts and is currently studying at Woosong Information College. He legally changed his first name from Woong-jae to Si-wan before his debut. Im plays the violin and guitar.

While attending Chin Chin Song Festival in Busan, Star Empire recruited him as a trainee. He entered a nine-membered group called "Children of Empire", where he trained for three years. In 2009, they group started doing guerilla performances around the country. On January 7, 2010, the group debuted with their EP, Nativity under the name ZE:A.

Im joined the cast of the period drama Moon Embracing the Sun (2012), playing the young version of Heo Yeom. Moon surpassed 40% ratings and gained "national drama" status, launching Im to fame. Later that year, Im was cast in the revenge drama Man from the Equator, playing a cold young man that has a sad story behind his negative outlook on life. This was followed by the sitcom Standby (2012), and Drama Special Waiting for Love (2013).

In 2013, he starred in the courtroom drama film The Attorney, playing a young student Activist detained and tortured by the police on accusations of being a communist sympathizer. The film was a critical and commercial success, selling more than 11 million tickets to become the 8th best-selling Korean film of all time. Im won Best New Actor at the Max Movie Awards and the Marie Claire Film Festival.

Im returned to television in 2014 and starred in two drama series. In Triangle, he played the youngest of three brothers separated at childhood, who grew up in a cold chaebol household. While in the webtoon adaptation and cable hit drama Misaeng, he reprised his role in Incomplete Life: Prequel as a former baduk player who learns to adapt to Korean corporate culture. Misaeng was a commercial hit and recorded high viewership ratings, with a peak of 8.4%; and was dubbed as a "cultural phenomenon". Im won the Excellence Award in acting at the 4th APAN Star Awards for his performance, as well as Best New Actor awards at the 9th Cable TV Broadcasting Awards and 51st Baeksang Arts Awards.

Im then took on his first big-screen leading role in war drama film A Melody to Remember (2016), playing a good-hearted soldier who is hoping for a miracle even in a desperate situation. He was also cast in the Chinese-South Korean web drama My Catman alongside Chae Soo-bin and Kim Myung-soo.

On July 11, 2017, he officially began his mandatory military Service and is scheduled for discharge on April 10, 2019. Im was chosen to be an assistant instructor for new recruits due to his good performance.