Handling organic waste is a significant challenge around the world. Using the generated organic waste as a source for biogas and biofuel production is one way to put the waste to use. Biochar is obtained as a byproduct during the production of biogas and biofuel. Pyrolysis, gasification, hydrothermal carbonization, and torrefaction are the commonly used methods to produce biochar. Biochar is a carbon-rich material and has been receiving increasing attention for its applications in various fields owing to its properties, such as stable structure, high surface area, pore volume, surface functionality, carbon content, and cation exchange capacity. Owing to these properties, biochar has various geoenvironmental applications such as contaminant removal, carbon sequestration, and an alternate fuel source. Consequently, biochar contributes to solid waste reduction, climate change mitigation, sustainable practices, and a circular economy. However, the applicability of biochar is limited due to its low density, limited contaminant removal range, and high impurities. These limitations can be overcome by modifying the biochar through physical, chemical, biological, and magnetic modifications and compositing biochar with various materials. This review encompasses methods employed to modify the biochar to attain desired properties and the multifaceted applications of the modified biochar in the sustainable geoenvironmental engineering domain.