Free (FP) and bound phenolics (BP) were extracted from freeze dried (FD), oven dried (OD), as well as boiling treated (BT) Pleurotus eryngii samples. Free, bound, total phenolics were quantified using Folin–Ciocalteau assay. Qualitative and quantitative analysis of phenolic compounds were carried out using high-performance liquid chromatography (HPLC) coupled with diode array detector (DAD), and a total of 8 phenolic compounds were detected. Free phenolic contents followed the order: Freeze-dried free phenolics (FDFP) > Oven-dried free phenolics (ODFP) > Boiling-treated free phenolics (BTFP), and ranged from 95.42 (BTFP) to 442.50 (ODPF) μg gallic acid equivalents (GAE)/g dry weight (DW). Bound and total phenolic contents followed the order: FD > OD > BT, and ranged from 218.33 (BTBP) to 774.17 (FDBP) and 313.75 (BT total phenolics) to 1090.42 (FD total phenolics) μg GAE/g DW. Bound phenolics contributed 49.76% (OD), 69.59% (BT), and 71% (FD) of the total phenolic contents. All free and bound phenolic extracts were investigated for their antioxidant activities by 3 different assays, 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity, reducing power, and superoxide anion radical scavenging activity. FDFP showed strongest DPPH radical scavenging activity (IC50 at 32.61 μg/mL), ODFP showed strongest reducing power (IC50 at 26.31 μg/mL), and BTBP showed strongest superoxide anion radical scavenging activity (IC50 at 14.07 μg/mL).