The increased demand for circular economy and bio-based solutions to address waste issues has triggered increasing interest in alternative feedstocks for biorefineries. Among the alternative feedstocks, underutilized biomass sources, including fruit and vegetable waste, chitin in crustacean shells, dried distillers’ grains (DDGs), bread wastes, broken rice, and beetroot, are promising opportunities for the development of bio-based products. They possess significant amounts of carbohydrates, protein, and bioactives, which are amenable to the prospect of utilizing them to develop high-potential feedstocks convertible to biofuels, bioplastics, enzymes, organic acids, and high-end chemicals. Valorization of the same feedstocks not only reduces the environmental footprint of food and agro-food waste but also enhances resource utilization efficiency. Technical challenges such as composition heterogeneity, efficient pretreatment, and economically viable conversion procedures, however, need to be addressed. New generation biorefinery configurations, novel bioconversion technologies, and regulatory facilitation need to be developed to promote these alternative feedstocks’ sustainable use in the future.