Agricultural fiber-reinforced PLA composites have gained significant attention in recent times. However, they have low thermal stability, high moisture uptake and inferior mechanical properties, which limit their use for different applications. Use of clay as filler in agricultural fiber-reinforced PLA composites can overcome these challenges. This paper presents thermal, physical and chemical properties of kaolin from Buwambo deposits in Uganda before and after alkali modification with NaOH and Mg(OH)₂. Alkali modification involved treating raw clay with different concentrations of alkali (1, 2, 4%) at different soaking times (1 h, 3.5 h) and liquor ratios (20:1, 5:1). Modified and unmodified clay samples were analyzed by SEM, Ultimate analysis, FTIR, TGA, XRF and Loss on Ignition. Physical properties for unmodified kaolin were 3.9 ± 0.0% (moisture), 6.8 ± 0.1% (volatile) and 89.3 ± 0.3% (ash). SEM analysis disclosed that alkali pre-treatment of kaolin improved its micro-structure. The FTIR spectra showed that the main functional groups were Al–OH, Al-O and Si–O. DTG results showed two main peaks related to maximum mass loss during kaolin combustion. Peak temperature of unmodified clay (660.2 °C) increased with increasing residence time and liquor ratio during alkali pre-treatment. Burning of kaolin above peak temperatures produces metakaolin which enhances creation of ash residues that signify increasing thermal stability. The properties of kaolin used in this study indicate its suitability for use as fillers in fiber-reinforced PLA composites.