Brown beech mushroom (Hypsizygus tessellatus) cap extracts cut colon cancer cell viability to 48.37% at 500 μg/mL, while showing low toxicity toward normal human cells. The phenolic-rich extract triggered dose-dependent G1 cell-cycle arrest and cell death, and inhibited E. coli and S. aureus with inhibition zones up to 18.87 mm at the highest tested concentration.