Since making his professional debut in 2015, Lee has competed several times for ONE Championship to great success. He quickly established himself as the most prolific finisher in ONE history, and he has emerged as an incredibly exciting young talent to watch. In 2019, Lee made history. He knocked out Japanese legend Shinya Aoki to become the youngest-ever male mixed martial arts World Champion, standing along with his sister Angela as the only sibling mixed martial arts World Champions in history.