Developing hemp fibre composites for structural applications requires both reconsideration and optimisation of the transformation processes to obtain stable, high-quality fibre reinforcements. In this context, field retting remains an important issue because it is weather dependent and has not been completely mastered by the hemp industries. Retting can be achieved voluntarily to facilitate fibre separation and extraction from the stalks prior to mechanical decortication. However, retting can also be involuntary and result from climatic misfortune and unforeseeable events at the time of harvest. Therefore, this study aimed to quantify the influence of involuntary and non-controlled field retting on the physicochemical and mechanical properties of industrial hemp bast fibres. A wide spectrum of analytical techniques was applied, including colour spectrophotometry; morphological, microscopic (SEM), surface (EDX, roughness), biochemical (HPLC and pXRD) and thermogravimetric (TGA) analyses; dynamic vapour sorption; and tensile characterisation. The results indicate that retting induced a decrease in the average width of fibre elements after mechanical processing and a loss of pectic substances. We also observed a change in colour from yellow to dark grey, an increase in surface roughness and an increase in the decomposition temperature for the third mass loss region. A decrease in tensile properties at the scales of both single and technical fibres was also observed. Since no significant decrease in cellulose content was measured, this decay in mechanical performance was connected with both the significant degradation of hemicelluloses and a decrease in the fraction of crystalline cellulose that was quantified in this work.