In response to the growing need for sustainable solutions, this study critically examines the significance of thermochemical conversion techniques for producing biochar from leaves. These techniques, including pyrolysis, torrefaction, gasification, and hydrothermal carbonization (HTC), show promise for sustainable biochar production. They vary in peak temperatures, reaction times, and biochar yields, providing distinct avenues for biochar utilization. Leaves from various biomasses, such as almond, sugarcane, bamboo, banana, eucalyptus, corn, palm, ginkgo, and coconut, have undergone thermochemical conversion into biochar. The peak temperatures for pyrolysis, hydrothermal carbonization, torrefaction, and gasification of leaves are 600 C-degrees, 375 C-degrees, 350 C-degrees, and 500 C-degrees, respectively. Among these techniques, the highest reported yield (88%) was achieved with torrefaction on sugarcane leaves. Pyrolysis produces biochar with a carbon content of up to 90%. Torrefaction and gasification offer diverse pathways for biochar and gaseous fuel generation, while HTC, involving elevated temperature and pressure treatment, demonstrates the potential for carbon-rich biochar. Hydrothermal carbonization results in lower ash-content biochar suitable for solid fuel use, while torrefaction can enhance biochar properties for various applications. Beyond the technical specifics, this work underscores the broader implications of thermochemical conversion techniques. The study highlights their role in waste management, energy production, and soil enrichment, aligning with the goals of circular economy models.