This paper introduces application of genetic algorithm (GA) in designing air-termination system of the high-voltage open-air substation external lightning protection system (LPS). The air-termination system, as a part of the external LPS of high-voltage substation, is known to be cumbersome to design using traditional techniques, where optimization is often relegated to the trial-and-error approach. In addition, traditional methods are often associated with geometric constraints and various implementation difficulties. This paper proposes a novel approach which utilizes a combination of statistical LPS efficiency and GA in designing techno-economically optimal external LPS of open-air substations. Proposed approach offers a LPS designer unique and valuable assistance in optimally arranging elements of air-termination system for obtaining maximum lightning shielding effects with minimum total investments.