Despite the growing number of physicians who reduce clinical time owing to research, administrative work, and family responsibilities, the quality of care provided by these physicians remains unclear. To examine the association between the number of days worked clinically per year by physicians and patient mortality. This cross-sectional analysis was completed on a 20% random sample of Medicare fee-for-service beneficiaries 65 years and older who were admitted to the hospital with an emergency medical condition and treated by a hospitalist in 2011 through 2016. Because hospitalists typically work in shifts, hospitalists' patients are plausibly quasirandomized to hospitalists based on the hospitalists' work schedules (natural experiment). The associations between hospitalists' number of days worked clinically per year and 30-day patient mortality and readmission rates were examined, adjusting for patient and physician characteristics and hospital fixed effects (effectively comparing physicians within the same hospital). Data analysis was conducted from July 1, 2020, to July 2, 2021. Physicians' number of days worked clinically per year. The primary outcome was 30-day patient mortality, and the secondary outcome was 30-day patient readmission. Among 392 797 hospitalizations of patients treated by 19 170 hospitalists (7482 female 39.0%, 11 688 male 61.0%; mean SD age, 41.1 8.8 years), patients treated by physicians with more days worked clinically exhibited lower mortality. Adjusted 30-day mortality rates were 10.5% (reference), 10.0% (adjusted risk difference aRD, -0.5%; 95% CI, -0.8% to -0.2%; P = .002), 9.5% (aRD, -0.9%; 95% CI, -1.2% to -0.6%; P < .001), and 9.6% (aRD, -0.9%; 95% CI, -1.2% to -0.6%; P < .001) for physicians in the first (bottom), second, third, and fourth (top) quartile of days worked clinically, respectively. Readmission rates were not associated with the numbers of days a physician worked clinically (adjusted 30-day readmissions for physicians in the bottom quartile of days worked clinically per year vs those in the top quartile, 15.3% vs 15.2%; aRD, -0.1%; 95% CI, -0.5% to 0.3%; P = .61). In this cross-sectional study, hospitalized Medicare patients treated by physicians who worked more clinical days had lower 30-day mortality. Given that physicians with reduced clinical time must often balance clinical and nonclinical obligations, improved support by institutions may be necessary to maintain the clinical performance of these physicians.