We propose a novel photonic harmonic mixer operating at frequencies up to the millimeter-wave (MMW) band. By combining a broadband fiber-wireless signal with high-order harmonics of a fundamental local oscillator in an optical frequency comb generator, frequency down-conversion can be implemented without using a costly ultra-broadband photodiode. It is theoretically shown that the down-conversion efficiency and the bandwidth of the mixer is highly dependent on the optical modulation indices and the fundamental frequency of comb lines. Down-conversion of a W -band (75-110 GHz) fiber-wireless signal is experimentally demonstrated. Moreover, the error vector magnitude (EVM) performance of a multigigabit quadrature phase-shift-keying (QPSK) signal at 62.5-, 82.5-, and 102.5-GHz carrier frequencies is studied to evaluate the down-conversion efficiency. The proposed photonic harmonic mixer can be a candidate for applications in high capacity fiber-wireless communication systems.