We investigate magnetic coupling between a monolayer of prototype single-molecule magnets Mn-12 and a ferromagnetic Ni(111) substrate through S, using density-functional theory (DFT) and a DFT+U method. Our DFT and DFT+U calculations show that the Mn-12 molecules favor antiferromagnetic coupling to the Ni substrate, and that they possess magnetic moments deviated from the magnetic moments of isolated Mn-12 molecules. We find that the magnetic easy axis of the Mn-12 on Ni (whole system) is dictated by that of the Ni substrate. The antiferromagnetic coupling is, dominantly, caused by superexchange interactions between the magnetic moments of the Mn and the Ni substrate via the S, C, and O anions. Our findings can be observed from x-ray magnetic circular dichroism or scanning tunneling microscopy.