Development of biocomposites is becoming a necessity due to environmental concerns. Polylactide is a matrix of choice, regarding its bio-sourced and compostable nature along with its mechanical properties. In this study, poly(L-lactide)/flax biocomposites were produced using thermoplastic resin transfer molding (TP-RTM) in one step process from the monomer, affording sustainable and fully compostable composite materials. The polymerization of L-lactide (L-LA) monomer was promoted by tin octoate, affording poly(L-lactide) (PLLA) matrices displaying monomer conversions superior to 96 % and mass-average molar masses above 140,000 g/mol. Flax fibres were used as received without any functionalization which is usually conducted to prevent the hydroxyl groups to conduct side reactions. Mechanical testing revealed bending modulus of 9.3 GPa and bending strength of 177 MPa comparable to existing literature values for PLLA/flax composites produced by other techniques. Digital microscopy analysis provided insights into impregnation quality and void characterization within the composites. In addition, the durability of these biocomposites was also evaluated via accelerated aging tests.