Isomerization induced by laser ionization in acetonitrile (CH3CN) was investigated using pump-probe spectroscopy in combination with ion-ion coincident Coulomb explosion imaging. We deduced five primary channels indicating direct C-C breakup, single and double hydrogen migration, and H and H2 dissociation in the acetonitrile cation. Surprisingly, the hydrogen-migration channels dominate over direct fragmentation. This observation is supported by quantum chemistry calculations showing that isomerization through single and double hydrogen migration leads to very stable linear and ring isomers, with most of them more stable than the original linear structure following ionization of the parent molecule. This is unlike most molecules investigated previously using similar schemes. By varying the delay between the pump and probe pulses, we have also determined the time scales of the corresponding dynamical processes. Isomerization typically occurs in a few hundred femtoseconds, a time scale that is comparable to that found for H and H2 dissociation and direct molecular fragmentation.