The identification of new photocatalytic pathways expands our knowledge of chemical reactivity and enables new environmentally friendly synthetic applications. However, the development of miniaturized screening procedures/platforms to expedite the discovery of photochemical reactions remains challenging. Herein, we describe a picomole‐scale, real‐time photoreaction screening platform in which a handheld laser source is coupled with nano‐electrospray ionization mass spectrometry. By using this method, we discovered an accelerated dehydrogenation pathway for the conversion of tetrahydroquinolines into the corresponding quinolines. This transformation is readily promoted by an off‐the‐shelf [Ru(bpy)3]Cl2⋅6 H2O complex in air at ambient temperature in direct sunlight, or with the aid of an energy‐saving lamp. Moreover, radical cations and trans‐dihydride intermediates captured by the screening platform provided direct evidence for the mechanism of the photoredox reaction.