Supercritical fluid extraction using CO2 (SCFE-CO2) was used to examine extraction efficiency of bioactive compounds from irregular-shaped Lion's Mane (LM) mushroom by-products. Optimal SCFE-CO2 conditions (46.38 °C, 100 bar, and 0.99 mL/min of EtOH flow rate) for maximum yield (a mass fraction of 11.64%), ergosterol (3.74 mg/g dried LM), total phenolic content, TPC (815.93 µg GAE/g dried LM), and antioxidant activity (42.97 µg TE/g dried LM) were determined using following parameters: temperature (40 °C to 60 °C), pressure (100 bar to 300 bar), and EtOH flow rate (0 mL/min to 1 mL/min). EtOH flow rate was a key factor in raising extraction yield and quality. SCFE-CO2 was equally effective compared to conventional extraction (Maceration) for LM, as there were no significant differences (p ≥ 0.05) in yield, ergosterol content, TPC, or antioxidant activity. This green extraction technology could be an excellent alternative method for recovering bioactive compounds from medicinal mushrooms. Display omitted •SCFE-CO2 was used to extract bioactive compounds from Hericium erinaceus mushroom.•Optimum conditions for SCFE-CO2 was 46.38 °C, 100 bar, and 0.99 mL/min of EtOH.•EtOH flow rate was a major factor that could improve extraction yield and quality.•SCFE-CO2 had an efficiency that could be used instead of conventional extraction.