Freeze-dried stems and caps of six cultivated mushroom species, namely Agaricus bisporus (white) Agaricus bisporus (brown), Lentinula edodes, Pholiota nameko, Pleurotus eryngii and Pleurotus ostreatus were subjected to ethanolic extraction. The obtained extracts were analyzed in terms of total phenolics content, total flavonoids content and antioxidant capacity, and the results were calculated per gram of mushroom dry weight and extract dry weight. The LC–MS technique was applied to determine the profiles of phenolic acids. The amount of total phenolics in the stems (per the fruiting bodies’ dry weight) ranged from 1.09 ± 0.09 mg/g (P. ostreatus) to 4.02 ± 0.20 mg/g (the white A. bisporus), whereas in the caps it ranged from 1.49 ± 0.07 mg/g (P. nameko) to 6.22 ± 0.21 mg/g (the brown A. bisporus). The total flavonoid content in the stems (per the fruiting bodies’ dry weight) varied from 0.46 ± 0.05 mg/g (P. ostreatus) to 1.72 ± 0.02 mg/g (the brown A. bisporus), and in the caps it ranged from 0.48 ± 0.01 mg/g (P. ostreatus) to 1.93 ± 0.05 mg/g (the white A. bisporus). The antioxidant potential measured with the FRAP assay showed that the caps displayed higher activity compared to the stems. However, in the case of the DPPH assay performed on A. bisporus, this relationship was inverted. Different species contained varied concentrations of phenolic acids. P. eryngii caps contained the highest amount of 3,4-DHBA, L. edodes caps were the richest source of caffeic acid, whereas the highest amount of syringic acid was observed in L. edodes stems. The caps of P. nameko contained the highest amounts of p-coumaric and t-cinnamic acid, as well as 4-hydroxybenzaldehyde.