The effects of filler content and methacrylate acid (MAA) treated Nypa fruticans husk (NFH) on the mechanical properties, X-ray diffraction (XRD), thermogravimetric analysis (TGA), and morphology of NFH regenerated cellulose (RC) biocomposite films were investigated. Ionic liquid containing 8 wt% of lithium chloride (LiCl)/N,N-dimethylacetamide (DMAc) was used to dissolve microcrystalline cellulose (MCC) and NFH to produce NFH RC biocomposite films. Methacrylate acid was used as a modifying agent on the NFH to promote better mechanical and thermal properties for the resulting NFH RC biocomposite films. The results showed that the tensile strength, Young’s modulus, crystallinity index (CrI), moisture content, and thermal stability of the untreated NFH RC biocomposite films increased with increasing NFH content up to 3 wt% and decreased with further increments. The MAA-treated NFH showed improved tensile strength and Young’s modulus compared with the untreated NFH RC biocomposite films. The presence of MAA enhanced the crystallinity index (CrI), moisture resistance, and thermal stability of the NFH RC biocomposite films. Good interfacial interaction between the NFH and RC matrix was proven by scanning electron microscopy (SEM).