Lee began working for SM as early as 1998 as a part-timer and officially joined the agency in 2005 as an A&R representative, before later becoming head of the production division.

A member of the company's core organization of A&R, he has been responsible for the songs and concepts of SM artists such as BoA, Girls' Generation, Shinee, f(x), Exo, Red Velvet, and NCT.

Born on November 3, 1979,[6] Lee Sung-soo graduated from the Department of International Trade at Hankuk University of Foreign Studies.

[8] Lee began working for SM Entertainment as early as 1998 while attending university as a part-timer focused on various tasks, such as identifying fan club trends.

[7][9] He developed an interest in A&R[10] and attended the SM Academy, where he studied computer composition and arrangement, stating that he should "know how to make music to be good at A&R".

[14] He had first started teaming up with foreign composers in the early 2000s, but initially found it difficult due to the low profile of K-pop at the time.

Following the release of several hit songs by SM artists in 2009, he began to be invited to songwriting camps hosted by foreign music publishers, and brought the idea back to South Korea.