Isomerization is a fundamental process usually initiated in molecules via photoionization. Using intense femtosecond IR laser pulses, in combination with coincident Coulomb explosion imaging, we are able to reconstruct the structure of the acetonitrile tri-cation after it undergoes isomerization and prior to fragmentation. Using momentum imaging, the Newton diagrams in tandem with native frame analysis reveal the signature of a triangular geometry. This indicates that upon photoionization to a triply ionized state, acetonitrile undergoes a clear structural change from its linear canonical form along with hydrogen migration. We further corroborate this with the aid of quantum chemistry calculations, which allow us to fully resolve this unique isomerization process. Understanding the underlying mechanisms behind such complex processes is relevant to biology and industrial applications.