This paper reports the use of cellulose microcrystals (CMCs) for improving fibre-matrix interface, mechanical, dynamic mechanical and thermal degradation behaviour of glass fibre reinforced epoxy composites. An ultrasonic treatment for 1 h was used to disperse CMCs (1–3 wt%) within an epoxy resin, which was subsequently infused through glass fabrics to develop hierarchical composites containing both macro and micro-scale reinforcements. It was observed that CMC dispersion in the epoxy resin was homogeneous at 1 wt% CMC and further increase in CMC concentrations led to linear increase in both agglomerate size and total agglomerated area. Addition of 1 wt% CMC to the composite matrix drastically changed the glass fibre-epoxy interface and led to a maximum improvement of 65% in interlaminar shear strength, 14% in tensile strength, 76% in flexural strength, 111% and 119% in fracture energy in tensile and flexural modes, 9.4% in impact strength, 13.5% in storage modulus, 21.9% in loss modulus and 13 °C in the glass transition temperature of composites. Therefore, the use of CMCs could be an industrially viable, economical and eco-friendly approach of developing hierarchical glass fibre composites with considerably improved performance.