With the awareness of the need for optimal and sustainable use of natural resources, hydrothermal treatment of biomass and biomass waste for energy and resource recovery has received increasing attention. In this context, hydrochars produced by hydrothermal treatment of biomass, which have been traditionally employed as catalyst supports or adsorbents, are now of considerable interest as precursors for activated carbons. Activated carbon produced from hydrochars has been widely used in applications such as energy storage, environmental remediation and resource recovery. Apart from savings in energy and resource conservation, hydrochars are of interest in activated carbon production because they possess unique attributes such as high density of oxygenated functional groups (OFG) and low degree of condensation which can be tailored to produce activated carbons with specific and desired characteristics. This review aims to integrate the current knowledge on hydrothermal carbonization of biomass waste in the context of resource and energy recovery. Specifically, we bring together recent advances made in this area through a systematic and critical review of the production of activated carbon from hydrochars. Mechanisms involved in the production of hydrochars with different surface characteristics in response to variations in the operating parameters are critically reviewed. The current knowledge gaps and challenges involved in the hydrothermal carbonization of biomass waste are critically evaluated with suggestions for further research.