AbstractMn(I) C−H functionalization of coumarins provides a versatile and practical method for the rapid assembly of fused polycyclic pyridinium‐containing coumarins in a regioselective manner. The synthetic strategy enables application of bench‐stable organomanganese reagents in bothphotochemical‐andthermal‐promoted reactions. The cyclomanganated intermediates, and global reaction system, provide an ideal testing ground for structural characterization of the active Mn(I) carbonyl‐containing species, including transient species observable by ultra‐fast time‐resolved spectroscopic methods. The thermodynamic reductive elimination product, solely encountered from reaction between alkynes and air‐stable organometallic cyclomanganated coumarins, has enabled characterization of a critical seven‐membered Mn(I) intermediate, detected by time‐resolved infrared spectroscopy, enabling the elucidation of the temporal profile of key steps in the reductive elimination pathway. Quantitative data are provided. Manganated polycyclic products are readily decomplexed by AgBF₄, opening‐up an efficient route to the formation of π‐extended hybrid coumarin‐pyridinium compounds.