Identification of impact events in sandwich structures is extremely challenging, because wave propagation in them can be highly complicated due to the interplay between facesheet and core materials. This becomes more evident when sandwich structures are exposed to harsh mechanical and/or thermal environments, which prevent the installation of sensing suites on their external surfaces. Therefore, conventional structural health monitoring techniques based on guided Lamb waves and their triangulations can often yield inaccurate and unreliable diagnostic outcome. To overcome this challenge, a novel diagnostic approach is proposed based on highly nonlinear solitary waves. Solitary waves are compactly-supported energy packets, which are generated by a balance of nonlinear and dispersive effects in nonlinear media, such as granular crystals. In this study, we fabricate a sandwich structure with a thick core that embeds arrays of granular crystals. Upon the external impact on the facesheet, granular crystals transfer energy and momentum of Lamb waves to highly nonlinear solitary waves. The solitary waves are recorded in real-time using embedded piezoelectric discs positioned at the tip of granular crystals. Based on the measured time-of-flights and disintegrated mode shapes of the solitary waves, we identify the location of impact on sandwich structures. The proposed granular crystal-based sensing system has a potential as an effective impact monitoring tool for inspecting structural damages in critical areas of sandwich structures under harsh environments. Refereed/Peer-reviewed