Background: Forward head posture leads to resulting in posture instability and changes in spine alignment. To treat this condition, numerous studies have shown the effectiveness of joint mobilization. However, only a few studies have reported on the intervention location of joint mobilization or its effects on heart rate variability. Objectives: To investigated the effects of joint mobilization intervention at the lower cervical spine on pain, head posture, and heart rate variability (HRV) in patients with a forward head posture. Design: A randomized controlled trial. Methods: Twenty patients with forward head postures who met the study inclusion criteria were recruited. A randomization program was used to divide the subjects into experimental and control groups. Each group consisted of ten subjects. The intervention was performed for four weeks, three sessions per week, 15 min per day, and non-simultaneously throughout the 12-week study period. Results: A significant decrease in neck pain was shown in both the experimental and control groups. In contrast, significant changes in craniovertebral angle (CVA), cranial rotation angle (CRA), and HRV were found only in the experimental group. Conclusion: The mobilization of the cervical spine is an effective intervention for improving pain, head posture, and HRV.