The carboxylation of cellulose by maleic anhydride in the molten solvent-free and catalyst-free process gave a low degree of substitution (DS). The most of maleic anhydride was sublimated in its melting point and stirring became a problem. Therefore, the cellulose was functionalized with maleic anhydride for the first time by a planetary ball mill at ambient conditions in the absence of solvent and catalyst. The chemical structure, morphology and surface composition, particle size distribution, surface charge, and the degree of substitution of the functionalized cellulose were investigated. A reduction in particle size was demonstrated. The EDX analysis showed an increasing average C/O mass ratio. The zeta potential decreased from -10.2 mV for cellulose to -4.08 mV for functionalized cellulose. The DS was 3.10 ± 0.02, 3.23, and 3.24 mmol of carboxyl groups per gram of functionalized cellulose by the conductometry, back titration, and combined TG-DSC analysis, respectively. XPS C1s and O1s energies demonstrated cellulose functionalization. Moreover, TG-DTA plots demonstrated a slight decrease in the thermal stability of the functionalized cellulose. Water vaporization in a two-step process was observed for the functionalized cellulose. A glass transition and decarboxylation were detected in the DSC plot of the functionalized cellulose.