The valorization of spent mushroom substrate (SMS) is a critical step for the sustainability and development of a circular economy in the mushroom industry. Large amounts of SMS are produced annually but are currently of little value. Fungal cultivation of lignocellulosic material is a biological, low-cost pre-treatment method that enhances enzymatic digestibility through the degradation of lignin and hemicelluloses. In addition, through fungal growth and the degradation of lignin, SMS becomes a good source of bioactive compounds. Extraction of bioactive compounds and cellulose saccharification are potential directions for the valorization of SMS. The aim of this study was to investigate the extraction of bioactive compounds with antioxidant activity from SMS of two Pleurotus spp. strains and evaluate the enzymatic digestibility of raw and extract-free SMS. Soxhlet and reflux extraction, ultrasound-assisted extraction (UAE), and subcritical water extraction (SWE) were used to extract various bioactive compounds. The results demonstrated that UAE and especially SWE are time-efficient extraction methods for the extraction of bioactive compounds from SMS. Six phenolic acids were identified in extracts from SMS through UV-HPLC, with vanillic, gallic, and chlorogenic acids showing the overall highest concentrations. The highest concentrations for vanillic and gallic acids were found in SWE and for chlorogenic acid in Soxhlet extraction with a 40:60 ethanol/water mixture. A strong positive correlation was found with ferric ion reducing power (FRAP) for both total phenolic compounds and caffeic acid, indicating that the observed antioxidant activity is largely contributed by phenolic compounds. The enzymatic digestibility was evaluated through analytical enzymatic saccharification and revealed an up to four-fold increase in digestibility after extraction of bioactive compounds. Results indicate that bioactive compounds can be a factor inhibiting enzymatic saccharification.