Anterior external fixation (EF), as the primary treatment for unstable pelvic fractures, is performed with patients in the supine position. In most cases, however, definitive surgery for posterior fixation is performed first in the prone position without EF. We report the case of a patient with unilateral and vertically unstable pelvic fracture whom we had treated with minimally invasive spinopelvic fixation, with retention of the anterior EF in a lateral position. Reduction of the residual displacement was performed with percutaneous spinal instrumentation, and acceptable reduction was achieved. At the 13-month follow-up, the functional outcome, calculated using the Majeed Score, was 87 points. The plain radiograph showed good bone union, except for the right superior pubic ramus. The radiological outcome, measured using the Matta rating, was excellent. Thus, retaining the EF facilitates safe and accurate reduction without major surgical complications and may offer surgeons an additional management option for such fractures.