This study explores the hybridizing effect of mechano-chemical activated fly-ash (FA) in sisal fiber reinforced polymer composites. Activation and resistance against agglomeration of FA has been achieved by modifying it with 2, 4, and 6 wt.% of the cetyltrimethylammonium bromide (C‑tab). FA activation with C-tab and particle size reduction to nano-level (<1µm) have been appropriately achieved with a planetary ball milling and the same has been confirmed from the dynamic light scattering technique. The hybrid composite containing 25 wt.% of sisal fiber and 5 wt.% of (6 wt.% C-tab) treated FA shows much improved tensile (40.12 MPa), flexural (53.27 MPa), and impact strengths (0.75 kJ/m2) than that of neat PP and composite reinforced with only 30 wt.% of sisal fiber. This increase in tensile and flexural strength was 30.54% and 48% higher than neat PP. Maximum notched impact strength of 0.80 kJ/m2 have been reported by hybrid composite containing FA treated with 2 wt.% of the C-tab. Micromechanical modelling using a combination of rule of mixture and inverse rule of mixture separately with Halpin-Tsai predicted a value close to the experimental Young’s modulus. DSC studies showed an increment in the composite's crystallinity upon fiber addition. Morphological analysis of the hybrid composite revealed good wettability of reinforcing fiber and FA within the matrix, whereas TGA showed an improved thermal stability of the composites.