In this study, the effect of Poly(ethylene-co-(acrylic acid)) and maleic anhydride on cellulose and lignin-filled EPDM elastomers was investigated. Poly(ethylene-co-(acrylic acid)) and maleic anhydride were added in certain amounts (3.5 phr) to determine their chemical, thermal, rheological, mechanical, and morphological properties. With the addition of Poly(ethylene-co-(acrylic acid)) and maleic anhydride, higher thermal stability and slower vulcanization with retarded ts2 and t90 were seen. Improved elongation and tear strength were also observed. However, maleic anhydride-added cellulose and lignin-filled EPDM had a higher tensile strength value since it formed adequate chain mobility and had good interfacial compatibility that made stress transfer easy. Despite some immiscibility phases (especially after maleic anhydride) and microvoid structures, no considerable aggregation was observed in all elastomers. In conclusion, Poly(ethylene-co-(acrylic acid)) and maleic anhydride on cellulose and lignin-filled EPDM elastomers can be recommended for mechanical, rheological, and ecological benefit to industries that are interested in biopolymer-filled EPDM elastomers.