Magnetic circular dichroism (MCD) is demonstrated for plasmonic bimetallic nanoparticles. They have an Ag–Au core–shell configuration, which can be synthesized using a seeded growth technique. In the seeded growth process, when the amount of Au (as coating metal) added to the Ag core is small, both ordinary (lower-energy) and extraordinary (higher-energy) modes of localized surface plasmon resonance (LSPR) are detected in optical extinction spectra, suggesting successful formation of well-defined core–shell morphology with a rather thin shell (= AgAu alloy) thickness. Interestingly, the related MCD, showing a typical bisignate magnetoplasmonic response, is more distinct than the extinction signal in the ordinary LSPR mode, but strong damping is found upon thin shell formation. Hence, such bimetallic nanosystems have an advantage of systematic tuning of their magnetoplasmonic behavior, but may not be favorable to obtaining large magneto-optical (MO) activity or MCD amplitudes. The damped MCD response is discussed from a viewpoint of spectral inhomogeneity.