A new simple method to prepare acidic mesoporous sulfonated silica in situ without any surfactant is reported. The prepared catalysts were characterized by XRD, nitrogen adsorption, thermogravimetric analysis (TGA), and elemental analysis. The concentration of –SO3H groups was measured by titration, and FTIR was applied to confirm the successful incorporation of the strong acid sites. The in situ sulfonated silica (ISS) has a large surface area with a narrow range of mesoporosity and good thermal stability. Moreover, the preparation method has the advantage of easy control of the acid concentration and versatile synthesis. The ISS was used in catalytic hydrolysis and showed remarkable reusability even under very harsh conditions. The butylation of phenol was conducted over ISS to utilize the mesoporosity of the catalysts. Finally, the ISS catalysts might be used in various acid catalyses owing to the advantages of their simple/inexpensive preparation, reusability, and mesoporosity.