The objective of this study was to obtain extracts from the mushroom Agaricus brasiliensis using supercritical fluid extraction (SFE) with pure CO2 and with CO2 plus 2.5%, 5.0% and 10.0% (w/w) of ethanol as co-solvent. In order to evaluate the high-pressure method in terms of process yield, extract composition and biological activity, low-pressure methods, such as maceration with ethanol (Mac), Soxhlet (Sox) with different organic solvents, and hydrodistillation (HD), were also applied to obtain extracts. The SFE conditions were temperatures of 313.15 K, 323.15 K and 333.15 K and pressures from 10 to 30 MPa. The SFE kinetics was investigated through the overall extraction curve (OEC). The extracts obtained by Sox with water and ethanol showed the best results for the global extraction yield. The best conditions in the studied range to obtain high yields using pure CO2 resulted to be 30.0 MPa and 323.15 K. The antioxidant potential of the extracts was evaluated by the DPPH method and by the Folin–Ciocalteau method. Maceration extract presented low yield, but good results of antioxidant activity by DPPH assays and total phenolic. The antimicrobial activity of the extracts was also studied. The main identified compounds in the extracts were linoleic and palmitic acids.