Experimental wood–plastic composites (WPCs) were made so that they matched the manufacturing process, dimensions, and water absorption of some commercial decking boards. WPC samples from selected formulations were divided into two identical groups. The first group was exposed in exterior conditions in Vancouver, British Columbia, and Hilo, Hawaii, at sun and shadow sites. Water absorption and biological activity were monitored by field inspection, density change measurement, and optical and scanning electron microscopy. The second group was used for soil block culture testing performed according to AWPA E10 (or ASTM D1413). Specimens were conditioned by immersion in water at room and elevated temperatures. Results of fungal decay activity are reported as specimen weight loss or corresponding density decrease. Observed density changes during field exposure and soil block culture testing are compared. Samples exposed to aggressive exterior conditions underwent decay, which was detected by microscopic inspection of board cross sections and calculated density decrease. Fruiting bodies of brown-rot decay fungi (Dacryopinax spathularia) were found on some sample surfaces during field inspections. The decay process of tested materials in the field seemed to require an initiation period dependent on exposure site. The shortest initiation time and the most aggressive environment for decay of WPC samples were found at the sunny site in Hilo. Laboratory soil block culture testing showed weight loss and density decrease of experimental WPCs to depend on conditioning. Correlations between laboratory test results and WPC performance in the field are described.