The growing volume of plastic waste presents a significant environmental challenge, necessitating innovative solutions for its effective management and reuse. This study investigates the optimization of waste plastic incorporation in asphalt concrete mixtures to enhance Marshall Stability and Flow properties, utilizing a Simplex Lattice Mixture Design. The waste plastics used are High-density polyethylene, polyethylene terephthalate, and polypropylene. The experimental design involves varying the proportions of three different types of waste plastics at concentrations of 0.25%, 0.5%, 0.75%, and 1.0% by weight of the asphalt concrete to determine their impact on the mixture’s performance. Marshall Stability and Flow tests are conducted to evaluate the mechanical properties of the modified asphalt concrete mixtures. Regression analysis is employed to develop predictive models that correlate the plastic composition with the Marshall Stability and Flow values, facilitating the optimization of the mixture design. It was identified the optimal mixture is with 1.0% PET which yielded the maximum Marshall stability and minimum Marshall flow values. ANOVA results underscore the significance of PET in influencing both Marshall Stability and Flow, indicating its crucial role in enhancing the mechanical properties of the mixture. The results of this study provide valuable insights into the potential of waste plastics as sustainable additives in asphalt concrete, offering a promising avenue for improving pavement performance and promoting environmental sustainability.