This research work attempts, for the first time, to use sisal fibres and microcrystalline cellulose (MCC) in combination for developing multi-scale cementitious composites with improved strength, modulus as well as fracture energy. MCC (0.1 %–3.0 wt.%) was first dispersed in water with the help of Pluronic F-127 surfactant (20 % of MCC wt.) using ultrasonication and subsequently, the MCC suspensions were added to cement-sand mixtures containing sisal fibers (0.25–2 wt.% of cement). Visual observation, UV–vis analysis and optical microscopy suggested that ultrasonication for only 15 min led to homogeneous MCC suspensions without significant sedimentation and Pluronic significantly reduced the total agglomerated area and the average MCC particle size in the suspensions. The use of up to 2 wt.% sisal fibres (without MCC) increased the fracture energy of cementitious composites by 351 % (after 28 days hydration) but reduced the compressive and flexural strengths by 38 % and 13 %, respectively. On the contrary, MCC (without sisal fibres) could strongly improve the compressive strength (20.5 % using 0.1 wt.% MCC) but was not found so effective in improving the fracture energy (improved by 29 % using 0.1 wt.% MCC). However, multi-scale reinforcements containing 0.1 wt.% MCC and 0.25−0.5 wt.% sisal fibres led to improvements of up to 18.4 %, 30.1 %, 30 % and 100 % in the compressive strength, flexural strength, flexural modulus and fracture energy of composites, presenting distinct advantages over only sisal fibre or MCC-based reinforcements. The results indicated the formation of higher amount of hydration products in multi-scale composites due to the positive effect of MCC on cement hydration. Superior hydration in the multi-scale composites resulted in a denser microstructure with a lower pore size and an improved fibre-matrix interface, improving significantly the strength, modulus and fracture energy.