In 2011, he formed a project group with label mate Kim Jungmo of TRAX called M&D (meaning "Midnight and Dawn" or "Miari and Dangae-dong", after the members' hometowns). They released their debut single, "Close Ur Mouth", in June. Heechul wrote the lyrics and directed the music video for the song, while Jungmo produced and performed the instrumental. Their first live performance was at Mnet’s summer awards ceremony 20’s Choice on July 7.
Kim Hee-chul carried out mandatory military service between 1 September 2011-30 August 2013. He served as a public service worker for 23 months after undergoing four weeks of basic training at the Nonsan military camp in Chungcheongnam-do.[15] At the time that he enlisted, he had been appearing on KBS's Secret (시크릿) as well as the "Radio Star" segment of MBC's Golden Fishery, for which bandmate Kyuhyun took over the role.[16]
Shortly after he had completed his military service, he featured on Kim Jang-hoon's single "Breakups are So Like Me". During live performances of the song on Music Bank and Show! Music Core, his part was performed by bandmates Yesung, Eunhyuk and Shindong.[17]
In 2014, he appeared on the "Global Edition" spinoff of MBC reality television programme We Got Married, as an on-screen couple with Taiwanese girl group member Puff Guo.[18][19] That year, he also played the role of a detective in the tvN drama Flower Grandpa Investigation Unit.[20]