Previous studies in patients with atrial fibrillation showed that a history of heart failure (HF) could negatively impact anticoagulation quality, as measured by the average time in therapeutic range (TTR). Whether additional markers of HF severity are associated with TTR has not been investigated thoroughly. We aimed to examine the potential role of HF severity in the quality of warfarin control in patients with HF with reduced ejection fraction. Data from the Warfarin versus Aspirin in Reduced Cardiac Ejection Fraction Trial were used to investigate the association between TTR and HF severity. Multivariable logistic regression models were used to examine the association of markers of HF severity, including New York Heart Association (NYHA) class, Minnesota Living with HF (MLWHF) score, and frequency of HF hospitalization, with TTR ≥70% (high TTR). We included 1,067 participants (high TTR, N = 413; low TTR, N = 654) in the analysis. In unadjusted analysis, patients with a high TTR were older and less likely to have had strokes or receive other antiplatelet agents. Those patients also had lower NYHA class, better MLWHF scores, greater 6-minute walk distance, and lower frequency of HF hospitalizations. Multivariable analysis showed that NYHA class III and/or IV (Odds ratio [OR] 0.68 [95% confidence intervals [CIs] 0.49 to 0.94]), each 10-point increase in MLWHF score (i.e., worse health-related quality of life) (OR 0.92 [0.86 to 0.99]), and higher number of HF hospitalization per year (OR0.45 [0.30 to 0.67]) were associated with decreased likelihood of having high TTR. In HF patients with systolic dysfunction, NYHA class III and/or IV, poor health-related quality of life, and a higher rate of HF hospitalization were independently associated with suboptimal quality of warfarin anticoagulation control. These results affirm the need to assess the new approaches, such as direct oral anticoagulants, to prevent thromboembolism in this patient population.