This study investigates the mechanical, thermal, and surface characteristics of acrylonitrile–butadiene–styrene (ABS) composites reinforced with bentonite and bis(2-ethylhexyl) phosphate (DEHPA)-modified bentonite. The addition of bentonite significantly reduced the tensile strength and elongation at break due to the filler's inherent brittleness. Pure ABS exhibited a tensile strength of 42.2 MPa, which decreased by 16.1% with the incorporation of 20% bentonite (ABS1). Impact strength tests showed that the inclusion of bentonite lowered impact resistance, particularly in notched samples. Bentonite also increased the hardness and density, reflecting improved rigidity but reduced flexibility. Surface gloss changed from semigloss to matte, and FT-IR spectroscopy confirmed the successful integration of bentonite. Water absorption tests revealed that DEHPA-modified bentonite composites had higher absorption over time, highlighting the modification's effect on hydrophilicity. While bentonite improves certain properties, the balance between mechanical strength and flexibility needs to be further explored to meet the requirements of specific applications such as automotive components, electronics casings, and construction materials.