Here we present a simple fabrication method of in-plane graphene micro-supercapacitor using flash light irradiation. Flash light reduction of graphene oxide (GO) is an efficient method of producing high quality reduced graphene oxide (rGO) in room temperature ambient conditions without the help of hazardous reducing agents (e.g. hydrazine, hydrogen iodide). The entire process is low cost and is capable of large scale fabrication. By carefully optimizing the photo-thermal reduction conditions and utilizing shadow mask in a similar manner as for photolithography, all-carbon, monolithic supercapacitor with interdigital finger structure can be fabricated in a simple irradiation step that occurs in less than one second. Microscale thinness of the device makes it highly flexible and thus useful for next generation electronics, such as wearable devices. The as-formed supercapacitor benefits from the in-plane structure, which allows full utilization of high surface area of graphene layers and rapid electrolyte diffusion that results in high energy and power densities.