Introduction The Infectious Diseases Society of America (IDSA) recommends a minimum of 5 days of antibiotic therapy in stable patients who have community-acquired pneumonia (CAP). However, excessive duration of therapy (DOT) is common. Define, measure, analyze, improve, and control (DMAIC) is a Lean Six Sigma methodology used in quality improvement efforts, including infection control; however, the utility of this approach for antimicrobial stewardship initiatives is unknown. To determine the impact of a prospective physician-driven stewardship intervention on excess antibiotic DOT and clinical outcomes of patients hospitalized with CAP. Our specific aim was to reduce excess DOT and to determine why some providers treat beyond the IDSA minimum DOT. Methods A single-center, quasi-experimental quality improvement study evaluating rates of excess antimicrobial DOT before and after implementing a DMAIC-based antimicrobial stewardship intervention that included education, prospective audit, and feedback from a physician peer, and daily tracking of excess DOT on a Kaizen board. The baseline period included retrospective CAP cases that occurred between October 2018 and February 2019 (control group). The intervention period included CAP cases between October 2019 and February 2020 (intervention group). Results A total of 123 CAP patients were included (57 control and 66 intervention). Median antibiotic DOT per patient decreased (8 versus 5 days; p < 0.001), and the proportion of patients treated for the IDSA minimum increased (5.3% versus 56%; p < 0.001) after the intervention. No differences in mortality, readmission, length of stay, or incidence of Clostridioides difficile infection were observed between groups. Almost half of the caregivers surveyed were aware that as few as 5 days of antibiotic treatment could be appropriate. Conclusions A physician-driven antimicrobial quality improvement initiative designed using DMAIC methodology led to reduced DOT and increased compliance with the IDSA treatment guidelines for hospitalized patients with CAP reduced without negatively affecting clinical outcomes.