Nontuberculous mycobacterial otomastoiditis is rare and can be easily confused with various different forms of otitis media. We describe the case of a 50-year-old woman who presented with left-sided chronic otitis media that had persisted for more than 1 year. It was not eradicated by standard antimicrobial therapy and surgical debridement. After appropriate antibiotic therapy for nontuberculous mycobacteria was added to the therapeutic regimen, the patient improved significantly and the lesion had healed by 6 months. Based on our experience with this case, we conclude that early bacterial culture and staining for acid-fast bacilli in ear drainage material or granulation tissue should be performed when standard antimicrobial therapy fails to eradicate chronic otitis media of an undetermined origin that is accompanied by granulation tissue over the external auditory canal or middle ear. Polymerase chain reaction testing is also effective for rapid diagnosis. Surgical debridement and removal of the foreign body can successfully treat nontuberculous mycobacterial otomastoiditis only when effective antimicrobial therapy is also administered.