Few studies have addressed the etiology and clinical outcomes of community-acquired pneumonia (CAP) treated in an ambulatory setting. We investigated the etiology by the culture of Mycoplasma pneumoniae, urine antigen testing of Streptococcus pneumoniae and Legionella pneumoniae, and DNA or RNA determination of eight kinds of respiratory virus DNA or RNA. An etiological diagnosis was made in 51.8% of 197 patients. The most common pathogens were M. pneumoniae (29.4%) followed by influenza virus A, parainfluenza virus, adenovirus, human metapneumovirus (9.6%), and S. pneumoniae (4.1%). Patients with mycoplasma infections were younger, less likely to have comorbidities, and less likely to have adequate sputum for gram stain and culture. Patients with viral infections were older and more likely to have poorly defined nodules on chest X-ray (CXR) or computed tomography (CT) scan. Among patients infected with M. pneumoniae, those with quinolones as initial prescriptions had shorter duration of fever after the initiation of antibiotics than patients with β-lactams, macrolides, or β-lactams + macrolides (p < 0.05). This study suggests that M. pneumoniae and respiratory viruses were the most frequent pathogens found in ambulatory adult CAP patients and quinolones were better than β-lactams, macrolides, or β-lactams + macrolides in the resolution of fever of M. pneumoniae pneumonia.