Molecule-in-matrix systems such as Langmuir films with photosensitizers and hydrogen evolution reaction (HER) catalysts may exhibit heterogeneity in photocatalytic activity across large-area films. Here, we introduce a confined illumination approach and simultaneous localized detection of light-driven hydrogen (H₂) evolution to directly correlate information on structural organization with catalytic performance of Langmuir–Blodgett and Langmuir–Schaefer films with incorporated [Mo₃S₁₃]^2– catalyst and a Ru-based photosensitizer. Derived from a coating of optical fibers used in scanning near-field optical microscopy (SNOM), the outside of the glass sheath of the microelectrode was modified with a silver coating. Coupling light into the coated glass sheath, local illumination with suppressed light loss, and visible light excitation directly beneath the microsensor apex could be achieved, resulting in a 3.4-fold higher photon flux density compared to the uncovered microelectrode. Using platinum-black-modified microelectrodes as sensitive H₂ microsensors, a bifunctional probe serving simultaneously as a local light waveguide and an H₂ microsensor, enables quantitative detection of photogenerated H₂ with spatial resolution. This is demonstrated here for films comprising a phospholipid matrix or a π-conjugated rigid molecular scaffold coembedding Ru-based photosensitizers and [Mo₃S₁₃]^2– catalysts. This enables not only the local quantification of H₂, but also the determination of the apparent quantum efficiencies (AQEs) of the different film architectures.