The tomography of photonic quantum states is key in quantum optics, impacting quantum sensing, computing, and communication. Conventional detectors are limited in their temporal and spatial resolution, hampering high-rate quantum communication and local addressing of photonic circuits. Here, we propose to utilize free electron-photon interactions for quantum state tomography, introducing electron homodyne detection with potential for femtosecond-temporal and nanometer-spatial resolutions. The detectable quantum information level depends on the electron-photon interaction strength. Our Letter opens avenues for free-electron-based photodetection utilizing the ultrafast, subwavelength, nondestructive nature of free electrons.