The coating is commonly used for corrosion protection of steel bridges due to its low cost and excellent workability. Visual inspections of the coating are quite important for maintaining corrosion protection performance. Additionally, gloss, color, and adhesion of the coating are often measured to support decision-making on repainting. However, a method for quantitatively evaluating the coating’s mechanical properties has not been established. Non-destructive evaluation of mechanical properties such as Young’s modulus can contribute to the quantitative assessment of the corrosion protection performance of existing steel structures. Therefore, this study focused on the nanoindentation method, which is a method for evaluating the quality of thin-film structures. Nanoindentation was well established for thin single-layer specimens with smooth surfaces, but this study examines the applicability of the C-5 system coating film which consists of fluorine, epoxy, and zinc layers. The fundamental loading test was conducted to measure the Young’s modulus of the C-5 system coating on the thin steel plate. In the nanoindentation method, a load-displacement curve is obtained by applying a constant load to the specimen with an indenter and then unloading it. This method determines Young’s modulus was calculated by the gradient of the unloading curve. Furthermore, the outdoor exposure test was conducted to reveal the change in mechanical properties of the C-5 system coating. Test specimens were put in the exposure test site located in Ogimi, Okinawa Prefecture, Japan. Okinawa Prefecture is a harsh environment for corrosion because of high temperatures and humidity as well as high precipitation. This study observed the initial Young’s modulus and the change after six months and one year, which revealed a degradation trend in the early stage of the life cycle. The thickness, gloss, color, and elongation of the coating were also measured during the exposure test, which relates to the change in Young’s modulus. The results of the fundamental test and the exposure test imply that Young’s modulus of the C-5 system coating can be measured but slightly change during the short-term exposure period.