In this work, injection moulded hemp fibre reinforced polylactide bio-composites of different fibre contents (0, 10, 20 and 30 wt%) were subjected to accelerated weathering of 12 h cyclic exposures of UV-light at 60 °C, water spray and condensation at 50 °C for 8, 16, 32, 48 and 64 cycles to study the changes in properties such as crystallinity, tensile, flexural, plane-strain fracture toughness (KIc) and strain energy release rate (GIc). The crystallinity of neat polylactide (PLA) was found to increase up to 50.6% after 64 cycles, whereas the crystallinity of composites of different fibre contents was found to increase in the range of 30.6 to 34.5% for 8 to 64 cycles. The overall mechanical properties (tensile, flexural, KIc and GIc) of the composites decreased as the number of cycles increased from 8 to 64. The crystallinity and the residual tensile strength, tensile modulus, tensile strain, KIc and GIc of the composites of 20 wt% fibres were found to be the highest after 64 cycles. In contrast, the residual flexural strength and flexural modulus of the composites of 30 wt% fibres were found to be the maximum after 64 cycles. Absorption of water, destruction of fibre integrity, degradation of PLA matrix, formation of cracks and pores were found to be the main causes of reduction in the mechanical properties of PLA bio-composites.