The Time in Therapeutic Range (TTR) is the gold-standard measure used to assess the quality of oral anticoagulation with vitamin K antagonists. However, TTR is a static measure, and International Normalized Ratio (INR) control is a dynamic process. Group-based Trajectory Models (GBTM) can address this dynamic nature by classifying patients into different trajectories of INR control over time. Objectives: The objective of this study was to assess the quality of INR control in a population-based cohort of new users of vitamin K antagonist with a diagnosis of atrial fibrillation using GBTM. Methods: We classified patients into different trajectories according to their propensity for being adequately anticoagulated over their first year of treatment using GBTM, and we evaluated the association between trajectories and relevant clinical outcomes over the following year. Results: We included 8024 patients in the cohort who fulfilled the inclusion criteria; the mean number of INR determinations over the first year of treatment was 13.9. We identified 4 differential trajectories of INR control: Optimal (9.7% of patients, TTR: 83.8%), Improving (27.4% of patients, TTR: 61.2%), Worsening (28%; TTR: 69.1%), and Poor control (34.9%; TTR: 41.5%). In adjusted analysis, Poor and Worsening control patients had a higher risk of death than Optimal control patients (hazard ratio: 1.79; IC 95%, 1.36–2.36 and hazard ratio: 1.36; IC 95%, 1.02–1.81, respectively). Differences in other outcomes did not achieve statistical significance, except for a reduced risk of transient ischemic attack in the Improving Control group. Conclusions: GBTM may contribute to a better understanding and assessment of the quality of oral anticoagulation and may be used in addition to traditional, well-established measures such as TTR.