Ulsan Hyundai Mobis Phoebus and Erie BayHawks (2013–2019) Lee returned to South Korea to begin his professional career. He was drafted first in the second round of the
2013 rookie draft by
Ulsan Hyundai Mobis Phoebus. With veteran stalwart
Yang Dong-geun already in his mid-thirties, Lee was given considerably more playing time, compared to other first-year rookies that season, as Mobis Phoebus won the championship. Lee enlisted for
mandatory military service in April 2015 and joined the
Sangmu team after completing basic training. He was discharged in January 2017. Lee joined the
2017 NBA G League draft and was picked by
Erie BayHawks. He averaged 2.5 points 0.9 rebounds and 1.1assists. Lee recorded 14.1 points, 2.8 rebounds, 3.6 assists, and 1.5 steals per game in the 2018-19 KBL season for Mobis Phoebus, winning both the championship and the regular season title. During the play-offs, he averaged 16.2 points and 3.6 assists and was named play-offs MVP. He also took part in the qualification of the team to the
2019 FIBA Asia Champions Cup in Bangkok and averaged 4.7 points, 4.7 rebounds and 4 assists at the championship.
Jeonju KCC Egis (2019–2020) With one year still left on his contract, Lee and teammate
Ra Gun-ah joined
Jeonju KCC Egis in a trade for four other players ahead of the 2019–20 season. He was unable to replicate his form from the previous season and was released as a free agent.
Goyang Orion Orions (2020–2022) Lee nearly signed with
Busan KT Sonicboom but joined
Goyang Orion Orions after contract negotiations broke down. After a strong performance in the pre-season KBL Cup competition, he quickly established himself as a mainstay in his new team. In the October game against his former team Ulsan Hyundai Mobis Phoebus, he put up a career-best 34 points, scoring 14 out of 15 free throws. He was voted into the
KBL Best 5 for the first time in his career. As his wife was about to give birth, he did not attend the awards ceremony and his Orion teammate
Heo Il-young received the award on his behalf. During the 2021–22 season, Lee was paired together with rookie guard Lee Jung-hyun. The backcourt pairing played a major role in the Orions qualifying for the playoffs and defeating Ulsan Hyundai Mobis Phoebus to advance to the semi-finals for the first time in five years; unfortunately they were defeated by eventual champions
Seoul SK Knights. The younger Lee would be nominated for the Rookie of the Year award while Lee Dae-sung himself finished the season as the top-scoring domestic player and was voted into the KBL Best 5 for a second consecutive time. He scored 20 or more points in 18 games during the regular season, only behind
Wonju DB Promy shooting guard
Heo Ung; it was the first time since the 2010–11 season any domestic player has scored 20 or more points in this many games.
Daegu KOGAS Pegasus (2022–2023) With the Orions facing uncertainty due to its ownership pulling out, Lee decided not to see out his contract and signed with
Daegu KOGAS Pegasus. ==National team career==