The synthesis of functional carbon materials with controllable morphology and structure using a simple, effective, and green process starting from biomass has been an attractive and challenging topic in recent years. After decades of technological development, high value-added biomass-derived carbon nanomaterials with different morphologies and structures prepared by low-temperature hydrothermal carbonization (HTC) have been gradually developed into a huge system covering different series in different dimensions, and are widely used in the fields of adsorption, electrochemical energy storage, and catalysis. However, due to a vague understanding of the fundamental structure–performance correlation and the absence of customized material design strategies, the diverse needs in practical applications cannot be well met. Herein, we reviewed the mechanism, modifications, and applications of the low-temperature HTC method for biomass. The synthesis mechanisms, structural designs strategies, and related applications of biomass-derived hydrochar are highlighted and summarized in different dimensions, including six major categories: zero-dimensional spherical structure, one-dimensional fibrous and tubular structure, two-dimensional lamellar structure, three-dimensional hierarchical porous structure, and special-shaped asymmetric structure. Then a sustainability assessment is conducted on the hydrothermal carbonization process. Finally, the controllable preparation of biomass-derived hydrochar is summarized and prospected for the application requirements in different fields.