Sung-yeol debuted with Infinite on June 9, 2010 with the extended play First Invasion. In July 2011, they released their first studio album titled Over the Top, which won the Disc Bonsang Award at the 26th Golden Disc Awards. In June 2013, they released their Japanese debut studio album Koi ni Ochiru Toki in two different editions. In 2014, Sung-yeol, L, and Sung-jong were launched as the group’s second sub-unit called Infinite F.
Sung-yeol officially made his acting debut in 2011 with a supporting role in SBS’s While You Were Sleeping. In 2013, he was cast as the male lead in the web drama Love for Ten: Generation of Youth. In 2014, he starred along with Kim Sae-ron and his bandmate Woo-hyun in the KBS2 coming-of-age series High School: Love On. In 2017, he led the cast of the KBS1 daily drama Love Returns.
Sung-yeol enlisted as an active-duty soldier for his mandatory military service in March 2019. He was discharged in October 2020. His younger brother Lee Dae-yeol is the leader of the boy group Golden Child.