As an actor, he is best known for starring in the film The Attorney (2013), and the television series Misaeng: Incomplete Life (2014), which were both successful at the box office and in the ratings.

[12] Im joined the cast of the historical drama Moon Embracing the Sun (2012), playing the young version of Heo Yeom.

[18] 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.

[24] Misaeng: Incomplete Life was a commercial hit, recorded high viewership ratings with a peak of 8.4%; and was dubbed a "cultural phenomenon".

[27][28] Im then took on his first big-screen leading role in the war drama film A Melody to Remember (2016), playing a good-hearted soldier who is hoping for a miracle even in a desperate situation.

[31] In a departure from his usual "nice guy" roles, Im plays a cunning conman in the crime caper movie One Line (2017).

[32] He deviated even farther from his clean-cut image in The Merciless (2017), a crime action film where he played an undercover cop who works for a drug-smuggling ring.

[45] It was finally released theatrically in South Korea on August 3, 2022[46] Im acting performance as the bioterrorist on the airplane earnings praise by critics and audience.

[56] The series was a commercial and critical success and his acting performance got rave reviews and he was nominated for Best Actor at the 60th Baeksang Arts Awards.

[57] On October 30, 2023, Narwal a smart robot vacuum cleaner company announced that it has selected actor Im Si-wan as their brand ambassador, who has the image of a sincere and strict child, as its model for its first campaign advertisement with the message 'Narwal who understands me'.

[58] On April 12, 2024, Nongshim announced that it has selected actor Im Si-wan as the new model for Baeksan mountain bottled water and will launch a new advertisement.

[61] On February 8, 2023, Im donated ₩10 million to help 2023 Turkey–Syria earthquakes through Hope Bridge National Disaster Relief Association.

[62] On July 17, 2023, Im donated ₩30 million to help those affected by the 2023 South Korea floods through to the Hope Bridge National Disaster Relief Association.