Assessment of ankle mobility is complex and of clinical relevance after an ankle sprain. This study develops and tests a biomechanical model to assess active ankle circumduction and its reliability. The model was then applied to compare individuals’ ankle mobility between injured and noninjured ankles after a sprain episode. Twenty patients with subacute unilateral ankle sprain were assessed at 4 weeks and 10 weeks after the injury. They underwent a clinical exam and an ankle circumduction test during which the kinematics were recorded with an optoelectronic device. A biomechanical model was applied to explore ankle kinematics. Reliability of the ankle circumduction tests were good to excellent (ICC of 0.55–0.89). Comparison between noninjured and injured ankles showed a mobility deficit of the injured ankle (dorsiflexion = −27.4%, plantar flexion = −25.9%, eversion = −27.2%, and inversion = −11.6%). The model allows a graphical representation of these deficits in 4 quadrants. Active ankle circumduction movement can be reliably assessed with this model. In addition, the graphical representation allows an easy understanding of the mobility deficits which were present in all 4 quadrants in our cohort of patients with subacute ankle sprain.