This paper presents a hybrid analysis and simulation method, capable of providing accurate, quantitative insight of noise in active mixers. The hybrid approach combines equations developed from an impulse response model with simulation of the time-varying impedance using periodic transfer function analysis. The hybrid method accounts for parasitic impedances and cyclostationary noise sources. It emphasizes time instants that contribute the most noise, and reveals the role of symmetry in reducing flicker noise. Excellent matching between the proposed method and traditional PNOISE simulation is demonstrated for a millimeter-wave active mixer.