During the transition from SM Entertainment to Good Entertainment, Lee debut as solo artist in November 2003, under the stage name of "M", being the first Shinhwa member to do so, as well as the first idol to do solo while still being in a group.
His first album, Un-Touch-Able, according to the January 2004 chart sold in excess of 100,000 copies, Two lead tracks were released for promotion: "Just One Night", the lead track from the album is a jazzy slow groove song and "Oh Rain".
His second single, “The M Style” was also quite popular as well, climbing up the charts just as quickly.
Though his songs manage to chart well, the album sales were not as strong as its two predecessors, with the album dropping off the Korean Monthly Chart rather quickly.
In 2007, Lee set up his own agency, M Rising Entertainment, to manage his solo activities.
Whilst he continued to perform as part of Shinhwa under Good Entertainment, until July 2010 where he became co-CEO with Eric Mun for Shinhwa activities.
Although he started out as a rapper and a singer, he stopped rapping as Shinhwa continued to release albums.
As a skilled dancer, he has choreographed some of Shinhwa's dances.
Where he performed his solo hits as well as cover versions of "Tonight" and "Cover Girl" by New Kids on the Block. After which, Lee debut as an actor in Wontak's Angel, released in summer of 2006.
Then for the remainder of the year he rejoined Shinhwa for the released of their 8th album State of the Art, and the associated promotion and Shinhwa 2006 Asia Tour: State of the Art concerts.
The album was ultimately a strong seller, placing 12th in MIAK's year-end charts with over 120,000 copies sold.
Lee kicked off 2006 with his first solo concert, M's Girl Friend, on 14 and 15 January at 88 Gymnasium in Hwagok-dong.
Lee released his first mini-album, The Sentimental Reason in February 2008 to coincide with Valentine's Day and White Day.