Importance: Statins decrease mortality in those with atherosclerotic cardiovascular disease (ASCVD), but statin adherence remains suboptimal. Objective: To determine the association between statin adherence and mortality in patients with ASCVD who have stable statin prescriptions. Design, Setting, and Participants: This retrospective cohort analysis included patients who were between ages 21 and 85 years and had 1 or more International Classification of Diseases, Ninth Revision, Clinical Modification codes for ASCVD on 2 or more dates in the previous 2 years without intensity changes to their statin prescription who were treated within the Veterans Affairs Health System between January 1, 2013, and April 2014. Exposures: Statin adherence was defined by the medication possession ratio (MPR). Adherence levels were categorized as an MPR of less than 50%, 50% to 69%, 70% to 89%, and 90% or greater. For dichotomous analyses, adherence was defined as an MPR of 80% or greater. Main Outcomes and Measures: The primary outcome was death of all causes adjusted for demographic and clinical characteristics, as well as adherence to other cardiac medications. Results: Of 347104 eligible adults with ASCVD who had stable statin prescriptions, 5472 (1.6%) were women, 284150 (81.9%) were white, 36208 (10.4%) were African American, 16323 (4.7%) were Hispanic, 4093 (1.2%) were Pacific Islander, 1293 (0.4%) were Native American, 1145 (0.3%) were Asian, and 1794 (0.5%) were other races. Patients taking moderate-intensity statin therapy were more adherent than patients taking high-intensity statin therapy (odds ratio [OR], 1.18; 95% CI, 1.16-1.20). Women were less adherent (OR, 0.89; 95% CI, 0.84-0.94), as were minority groups. Younger and older patients were less likely to be adherent compared with adults aged 65 to 74 years. During a mean (SD) of 2.9 (0.8) years of follow-up, there were 85930 deaths (24.8%). Compared with the most adherent patients (MPR ≥ 90%), patients with an MPR of less than 50% had a hazard ratio (HR; adjusted for clinical characteristics and adherence to other cardiac medications) of 1.30 (95% CI, 1.27-1.34), those with an MPR of 50% to 69% had an HR of 1.21 (95% CI, 1.18-1.24), and those with an MPR of 70% to 89% had an HR of 1.08 (95% CI, 1.06-1.09). Conclusions and Relevance: Using a national sample of Veterans Affairs patients with ASCVD, we found that a low adherence to statin therapy was associated with a greater risk of dying. Women, minorities, younger adults, and older adults were less likely to adhere to statins. Our findings underscore the importance of finding methods to improve adherence.