Flax fibres are widely used in the automotive sector to reinforce polyolefins, such as for dashboard and interior door panels. A promising option is poly-(lactid) (PLA), as it leads to higher mechanical properties and offers an additional end-of-life scenario following recycling: industrial composting. However, like other composite systems such as flax/polyolefin, flax/PLA composites are also sensitive to water. Here, a non-woven flax/PLA composite is aged under several conditions (50% RH/75% RH/98% RH/Immersion) until saturation. After ageing, all samples are reconditioned at 50% RH, and their residual properties are assessed. The presence of a critical relative humidity between 75% and 98% is highlighted, above which, increases in moisture content irreversibly decrease the composite’s mechanical properties. After ageing at 98% RH and in immersion, tangent modulus was reduced by 23.0 and 33.8% and ultimate strength by 26.7 and 37.4%, respectively, compared to reference materials. This decrease is mainly due to microstructure evolution in the form of increasing porosity. This microstructure evolution is induced by the swelling of flax fibres, which generates high local stresses, above what PLA can withstand. As a result, micro-cracks appear in the matrix, responsible for reduction in mechanical properties.