Background <p>The impact of adding cervical mobilization in Subacromial Impingement Syndrome (SIS) remains underexplored. This study investigated the effects of adding cervical to shoulder mobilization on pain, sensation, and function in patients with Subacromial Impingement Syndrome.</p> Methods <p>The study was registered at ClinicalTrials.gov (NCT06602206) on 30/07/2024. Participants (<i>n</i> = 45, aged 27–65) were randomly allocated into three groups (<i>n</i> = 15 each): a conventional treatment group (6 males, 9 females), a shoulder mobilization group (6 males, 9 females), and a cervical mobilization group (5 males, 10 females). Pain was assessed using the visual analog scale (VAS), pressure pain threshold (PPT) with a handheld dynamometer, tactile sensation with a two-point discrimination test using an esthesiometer, range of motion (ROM) and pain-free ROM with a goniometer, and functional status with the Disabilities of the Arm, Shoulder and Hand (DASH) questionnaire. All participants underwent a 15-session treatment program. Assessments were conducted before and after the intervention.</p> Results <p>All groups improved in pain, ROM, sensation, and function (<i>p</i> &lt; 0.05). The cervical mobilization group showed a greater reduction in VAS scores than the others (F = 5.251, <i>p</i> = 0.009). Pain-free ROM improved most in the cervical and least in the control group (F = 3.501, <i>p</i> = 0.039). PPT and tactile sensation increased significantly in the cervical group (F = 3.256, <i>p</i> = 0.048; F = 3.529, <i>p</i> = 0.038). Both mobilization groups improved more in DASH scores than controls (F = 3.933, <i>p</i> = 0.027). All outcomes had large effect sizes.</p> Conclusion <p>While all interventions were beneficial in SIS, cervical mobilization combined with conventional treatment and shoulder mobilization proved most effective in reducing pain, enhancing pain-free ROM, and improving sensory function. Minimal Clinically Important Differences values were established for all assessment tools.</p>