2020–present: Transition to lead roles
In 2020, Lee had his first lead role in the romantic comedy 18 Again, based on the 2009 American film 17 Again. His performance in the drama earned him the Best New Actor award in television at the 57th Baeksang Arts Awards and Best New Actor award at the 7th APAN Star Awards. Later that year, he had a starring role in Netflix's Sweet Home, adapted from the webtoon of the same name. He received a good response for portraying a cool-headed and cynical character.
In 2021, Lee appeared in Beyond Evil, directed by Shim Na-yeon, where he played younger self of Shin Ha-kyun's character. He also starred in KBS2's Youth of May, a drama set in 1980 during the Gwangju uprising, earned him a "Next Generation Melo-king" title for his spectacular performance in a melodrama genre. Later in July 2021, Lee was confirmed to join the tvN drama Melancholia with Im Soo-jung, airing in the second half of 2021. At the end of 2021, Lee became the MC of the 2021 KBS Drama Awards with Sung Si-kyung and Kim So-hyun.
In 2022, Lee starred in the web series Reincarnation Romance, a short web series advertising motion sickness pills. This is the third reunion with Go Min-si. In December, Lee returned to the small screen with the Netflix drama written by writer Kim Eun-sook titled The Glory, with Song Hye-kyo. In 2023, Lee chose Death's Game as his last project before joining the military because of the bond he made with director Ha Byung-hoon, which he worked with in 18 Again.