Endovascular thrombectomy (EVT) is the standard treatment for acute ischemic stroke (AIS), yet nearly 40% of patients fail to achieve complete revascularization on the first EVT attempt. In vitro1 and in silico2 studies have shown that thrombus stiffness influences thrombus-device interaction and retrieval efficacy. We prospectively designed an observational study to characterize the mechanical properties of EVT-retrieved thrombus, and assess pre-interventional predictors of thrombus mechanics, and investigate the relationship between thrombus stiffness and EVT success. To explore reproducibility of the mechanical evaluation procedure, properties of blood clot analogs were tested and the reproducibility within and between observers was determined. Clot contraction, together with unconfined compression and tensile to failure tests were performed and we demonstrated that inter-observer variability ranged from 5% for contraction experiments to 18% for compression experiments. Intra-observer variability was larger, ranging from 8% to 26%. We found that first-pass EVT showed lower success rates in the intracranial carotid artery (ICA) and M2 segment of the middle cerebral artery occlusions, with thrombi in the ICA that were significantly stiffer compared to thrombi in M2. These observations suggested that the most difficult thrombi to retrieve in the anterior circulation are the stiffest ones in proximal occlusions, and the softest thrombi in distal occlusions, indicating that different EVT strategies tailored on thrombus stiffness differences among occlusion sites could be appropriate for different vascular territories in the anterior circulation. We were able to demonstratethe reproducibility of the experimental appraoch and showed a clear relationship between thrombus mechanical properties, occlusion location and location-dependent first pass success rate with a clear route towards clinical application. Distal thrombi are significantly softer than proximal ones, qualifying occlusion site as a clinical surrogate for thrombus stiffness. The relationship between stiffness and recanalization success varies by occlusion site, underscoring the need for stiffness-specific EVT techniques and device designs.