Many new physics scenarios beyond the Standard Model often necessitate the existence of a neutral scalar H, which might couple to the charged leptons in a flavor violating way, while evading all existing constraints. Such scalars could be effectively produced at future lepton colliders like CEPC, ILC and FCC-ee, either on-shell or off-shell, and induce lepton flavor violating (LFV) signals. We find that a large parameter space of the scalar mass and the LFV couplings can be probed, well beyond the current low-energy constraints in the lepton sector. In particular, a scalar-loop induced explanation of the longstanding muon $g-2$ anomaly can be directly tested in the on-shell mode.