A composite polymer process aid is provided. The process aid includes one or more inorganic mineral fillers and one or functionalized polymers. The functionalized polymers include at least one base polymer functionalized with an epoxy, hydroxyl, β-keto ester, β-keto amide, or carboxylic acid reactive functional group. The composite polymer process aid is in the form of a plurality of individual particles. At least a portion of the individual particles include both inorganic mineral fillers and the functionalized polymers. The composite polymer modifier is formed by a process of: i) blending an aqueous solution, an aqueous dispersion, an aqueous suspension, an aqueous slurry, or a dry powder of the mineral fillers with an aqueous solution, an aqueous dispersion, an aqueous suspension or an aqueous emulsion of the functionalized polymers to form an aqueous blend, and ii) powderizing the aqueous blend to form the composite polymer process aid.