The quest for combining semiconducting with ferromagnetic properties has recently led to the exploration of Mn substitutions not only in binary (GaAs, CdTe), but also in ternary semiconductors such as chalcopyrites ABⅢX2Ⅵ. Here, however, Mn would substitute any of the two metal sites A or B. The site preference of Mn doping in CuMⅢX2Ⅵ chalcopyrite is crucial because it releases different type of carriers: electrons for substitution on the Cu sites, and holes for substitution on the MⅢ sites. Using first-principles calculation we show that Mn prefers the MⅢ site under Cu-rich and Ⅲ-poor conditions, and the Cu site under Ⅲ-rich condition. We establish the chemical potential domains for pure CuAlS2, CuGaS2, CuInS2, CuGaSe2, and CuGaTe2 stability. We show that the solubility of Mn on the MⅢ (Cu) site increases (decreases) as the Fermi level moves toward the conduction-band minimum (n-type conditions). It is further found that domains of chemical stability of all these chalcopyrites may be largely reduced by Mn incorporation.