To verify master curves obtained based on time–temperature superposition principle for wood–plastic composites (WPCs), a 220-day long-term creep test was conducted under an unconditioned environment. In this study, WPCs were made by extrusion with various formulations; using mountain pine beetle-attacked lodgepole pine flour and high-density polyethylene as raw materials, as well as maleated polypropylene as coupling agent. The results showed that the effect of naturally elevated temperature during the summer months caused additional increases in creep strain. The information obtained from the conventional creep study method may be insufficient to reflect the practical application. Comparisons between long-term data and the master curves showed that the master curves tended to overestimate the real creep strain of large specimens and that the deviation increased with time. The prediction of the master curve agreed more reasonably with the long-term data for coupled WPC products, whereas the master curves showed considerable overestimation for the uncoupled ones. In general, the master curves cannot precisely predict the long-term creep strain, but merely provide conservative estimations.