Kaolinite and the modified natural zeolite minazel plus (M+) were used as supplements in substrate used for the production of the medicinal mushroom Grifola frondosa. Growth stimulation, expressed as yield and biological efficiency, was observed when M+ (1%) was added. The production cycle was shortened by half as a result of the zeolites' ion-exchange ability, stimulation of enzyme activity, and water retain capacity. Inductively coupled plasma optical emission spectrometry of fruiting bodies showed the absence of heavy metals (arsenic, cadmium, and lead), whereas the concentration of calcium increased greatly and the concentrations of iron, magnesium, and zinc increased moderately under the influence of M+. Solid-state nuclear magnetic resonance showed a positive impact on the β-glucan ratio, which could have been caused by the epimerization reaction stimulated by zeolites. The functionality of the mushroom was evaluated through several antioxidant activity assays, and in all cases a positive effect was established: M+ was statistically more effective in comparison with kaolinite. A strong correlation was established between the antioxidative activity of cultivated fruiting bodies and the tested compounds (total phenolic compounds, carbohydrates, and minerals).