Cassava processing residues are renewable lignocellulosic resources with potential for sustainable composite development. This study evaluated the influence of sorbitol content on the thermogravimetric behaviour of chemically treated cassava cortex (CCT)-based materials. Five formulations were investigated: CCT100, SBT100 and composites containing 5, 15 and 25 wt.% sorbitol (SBTCCT5, SBTCCT15 and SBTCCT25). Thermogravimetric analysis was conducted with a NETZSCH STA 449C using 8.0 ± 1.5 mg specimens heated under nitrogen from 20 to 900 °C at 10 °C min−1. Analysis of the TGA and derivative data showed composition-dependent, multistage degradation. SBTCCT5 exhibited T5 = 314.4 °C and T10 = 333.8 °C, while SBTCCT25 gave T5 = 314.1 °C and the highest T10 = 342.0 °C. SBTCCT15 showed lower initial-degradation thresholds (T5 = 231.7 °C; T10 = 279.0 °C) but retained 16.36% mass at 800 °C, close to SBTCCT5 (16.55%). The principal derivative peak temperatures were approximately 370.6–458.2 °C across the formulations. The results demonstrate a non-monotonic effect of sorbitol concentration: thermal stability depends on the specific CCT:SBT ratio rather than increasing uniformly with sorbitol content. The comparatively high T5/T10 of SBTCCT5 and SBTCCT25 and the higher high-temperature residue of SBTCCT5/SBTCCT15 identify different formulation advantages. These findings support cassava cortex valorisation as a renewable component for bio-based material systems, while further mechanical, moisture and long-term thermal studies are required for application qualification.