Paranasal sinus osteomas are mostly asymptomatic; however, secondary mucocele can develop if they impede the natural sinus drainage. Such a mucocele can destroy the bone and extend into the adjacent structures. We report on an unusual case of frontal sinus osteoma in a 27-year-old patient, complicated by large secondary mucocele that eroded the bone and extended into the frontal lobe of the brain. Unexpectedly, the patient did not report any visual or other symptoms attributable to central nervous system deficit. The mucocele was completely resected through bifrontobasal osteoplastic craniotomy, whereas osteoma was evacuated in its entirety by both drilling and mobilizing. Open surgical approach remains the main treatment for complicated paranasal sinus osteoma, and radical removal of intracranial mucocele is mandatory to prevent the development of life-threatening infections. Although intradural extension of a secondary mucocele is extremely unusual, head and neck surgery specialists should take this severe complication in consideration.