This study evaluated the criterion-related validity of the Electronic Head Posture Instrument (EHPI) in measuring the craniovertebral (CV) angle by correlating the measurements of CV angle with anterior head translation (AHT) in lateral cervical radiographs. It also investigated the correlation of AHT and CV angle with the Chinese version of the Northwick Park Questionnaire (NPQ) and Numeric Pain Rating Scale (NPRS). Thirty patients with diagnosis of mechanical neck pain for at least 3 months without referred symptoms were recruited in an outpatient physiotherapy clinic. The results showed that AHT measured with X-ray correlated negatively with CV angle measured with EHPI (r = -0.71, p < 0.001). CV angle also correlated negatively with NPQ (r = -0.67, p < 0.001) and NPRS (r = -0.70, p < 0.001), while AHT positively correlated with NPQ (r = 0.390, p = 0.033) and NPRS (r = 0.49, p = 0.006). We found a negative correlation between CV angle measured with the EHPI and AHT measured with the X-ray lateral film as well as with NPQ and NPRS in patients with chronic mechanical neck pain. EHPI is a valid tool in clinically assessing and evaluating cervical posture of patients with chronic mechanical neck pain.