Time-reversibility, which is inherent in many physical systems, is crucial in tailoring temporal waveforms for optimum light–matter interactions. Among the time-reversible atomic systems, narrowband biphoton sources are essential for efficient quantum storage. In this work, we demonstrate time-reversed and fully time-resolved ultra-narrowband single-sided biphoton frequency combs with an average free-spectral range (FSR) of 42.66 MHz and an average linewidth of 4.60 MHz in the telecommunication band. We experimentally observe the fully time-resolved and reversible temporal oscillations by second-order cross correlation and joint temporal intensity measurements. The potential benefits of the time-reversed and fully time-resolved temporal oscillations from our source include enhancing the efficiency of quantum storage in atomic memories and maximizing the utilization of temporal information in multimode biphoton frequency combs. We further verify the heralded single-photon state generation from the multimode biphoton frequency combs by using Hanbury Brown and Twiss interference measurements. To the best of our knowledge, this 42.66 MHz FSR of our photon-pair source represents the narrowest among all of the different configurated biphoton sources reported to date. This ultra-narrow FSR and its 4.60 MHz linewidth provide the highest frequency mode number of 5786 and the longest coherence time among all the singly configurated biphoton sources so far. Our time-reversed and fully time-resolved massive-mode biphoton source could be useful for high-dimensional quantum information processing and efficient time–frequency multiplexed quantum storage toward long-distance and large-scale quantum networks.