Chemoselective photoredox fluorination is an appealing approach to access fluorinated fine chemicals such as active pharmaceutical ingredients, but most of the known procedures currently lack time-resolved mechanistic insights. We use nanosecond transient absorption spectroscopy and density functional theory (DFT) calculations to elucidate the elementary steps after irradiation in a photocatalytic fluorination procedure that we reported previously. Time-resolved optical spectroscopy suggests that direct reaction only occurs between the photoexcited anthraquinone (AQN) and Selectfluor®. We have observed spectroscopic evidence of a novel transient AQN–Selectfluor® species for the first time. Further studies by DFT calculations suggest that the AQN–Selectfluor® triplet exciplex formed by photoirradiation is responsible for initiating and sustaining the fluorination reaction.