Lee Sang-yoon was picked out of a crowd on the streets of Yeouido by his former agency Director at the age of 24. Making his entertainment debut in 2005 in a Hite Beer commercial, he went on to star in several television series. He won Best New Actor at the 2010 MBC Drama Awards for Home Sweet Home.
Lee earned recognition with his role in the family weekend drama My Daughter Seo-young (2012), which recorded a peak viewership rating of 47.6%, making it the highest rated Korean drama of 2013.
In 2013, Lee graduated from the prestigious Seoul National University with a degree in Physics. He had entered as a freshman in 2000, but his acting career and mandatory military Service had caused him to take several years of leave from SNU, resulting in his delayed graduation.
Lee starred in his first big screen leading role in the 2014 romance film Santa Barbara. The same year, he also played lead roles in the medical drama Angel Eyes and mystery thriller series Liar Game.
In 2015, Lee starred in romantic comedy series Second 20s.
In 2016, Lee starred in romance melodrama On the Way to the Airport. He had his second big screen leading role in the thriller film Insane.
In 2017, Lee starred in SBS' legal drama Whisper, playing an elite judge.
In 2018, Lee is set to star in fantasy melodrama About Time alongside Lee Sung-kyung.