Photonic millimeter-wave frequency multiplication based on external modulation is considered as an attractive solution for high-frequency and frequency-tunable millimeter-wave signal generation due to its simplicity and stability, in addition to the large frequency tunability, and high spectral purity of the generated millimeter-wave signal. A generalized approach to achieve microwave frequency-multiplying generation based on external modulation using a dual-parallel Mach-Zehnder modulator (DPMZM) is presented. A theoretical analysis is developed, and the operating conditions are given to achieve frequency quadrupling, sextupling, and octupling millimeter-wave generation. Finally, a novel scheme for photonic generation of frequency quadrupling millimeter-wave signal without wideband microwave phase shifter and optical filter is proposed. The impact of the finite extinction ratio (ER) and modulation index of DPMZM on the generated millimeter-wave signal is also evaluated.