We present our design method for a 1 m lightweight mirror in a space optical system. The mirror made of Zerodur® has pockets at the back surface and three square bosses at the rim. Metallic bipod flexures support the mirror at the bosses and adjust the mirror’s surface distortion due to gravity. Their dimensional parameters cannot be optimized independently from each other in a conventional design process, where the mirror’s optical performance is greatly influenced by the flexure configuration. With our method, the design problem is separated into two independent problems; mirror design and flexure design. Resources required to achieve the design goals are reduced by almost one order of magnitude in time. We implemented a multi-objective genetic algorithm to optimize the mirror design and satisfied the design goals. We also present a new adjustable bipod flexure as an optical compensator for the gravity-induced aberration, instead of using a monolithic bipod flexure.