Most mobile-type hip exoskeletons only assist on a sagittal plane with a single actuator per hip joint for compactness, lightweightness, and low cost. Even though a new concept of hip mechanism that allows assistance not only on a sagittal plane but also on a frontal plane only with a single actuator has been proposed recently, it has several limitations, such as having a small range of motion and being applicable only on a single gait-mode. Herein, we propose a novel design approach that is a well-combined procedure of target design with clinical walking data, multi-objective design problem definition, type synthesis of mechanism, and analysis of the obtained Pareto-front solutions. We take the hip spherical mechanism’s kinematic trade-offs into account. By utilizing a more complex type of 1-DOF(Degrees of Freedom) mechanism (6-bar Stephenson-III) and multi-objective optimization approach, we successfully enhanced both compactness and hip moment assistance in the single-actuator hip exoskeleton mechanism, and its performance was validated via benchtop experiment of its prototype.