Acid-treated montmorillonites (MMT) were used as catalysts of carvone isomerization to carvacrol. Mineral acids—sulfuric, hydrochloric, nitric acids and organic acids (acetic and chloroacetic)—were used for the acid treatment. Prepared materials were characterized by available characterization methods, namely XRD, EA, TPD, TPO, UV-Vis, laser light scattering and nitrogen physisorption. The structure of montmorillonite remained intact after treatment. However, TPD proved the increase of acidity of acid-treated materials comparing pure montmorillonite. All materials were tested in the isomerization of carvone, producing carvacrol as the desired product. The initial reaction rate increased using the materials in the row MMT-COOH < MMT-HNO3 < MMT-ClCOOH < MMT-H2SO4 < MMT-HCl, which is in accordance with the pKa of acids used for the treatment. The number of weak acid sites strongly influenced the selectivity to carvacrol. The optimal solvent for the reaction was toluene. Total conversion of carvone and the selectivity to carvacrol 95.5% was achieved within 24 h under 80 °C, with toluene as solvent and montmorillonite treated by chloroacetic acid as catalyst. The catalyst may be reused after calcination with only a low loss of activity.