The need for measuring the mobility of the first ray has been identified. The purpose of this study was to test the reliability and validity of a device built to measure the relative vertical displacement of the first ray. Twenty fresh frozen cadaver feet were sampled (mean age of donor was 70 ± 13 years). Dorsal mobility of the first ray was measured by device over three trials of repeated loading using 20 N, 35 N, 55 N and 85 N of force. Radiographs served as the criterion standard for validation of the device. Vertical displacement was measured from x-ray by a digitizing procedure that recorded movement of the first ray during loading. Reliability of the device assessed by intraclass correlation coefficients (ICC) was .98 with standard errors of the measurement calculated to be 0.35 mm. A correlation of agreement value of .97 was determined for the two methods of measurement. Analysis of variance testing found a significant interaction between force and method of measurement. Tukey post-hoc analysis found no difference (F < 1.70) between the two methods of measuring first ray displacement in cadaver specimens when the force applied did not exceed 55 N. At 85 N of force (F = 10.05), unwanted movement of the second metatarsal caused the device to overestimate the amount of displacement that occurred specific to the first ray. This measure of first ray mobility should help clinicians and researchers to better investigate foot pathology resulting from faulty mechanics of the first ray.