Numerous studies have focused on the mechanical control of solid structures and phase changes in molecular crystals. However, the molecular-level understanding of how macroscopic forces affect the molecules in a solid remains incomplete. Here we report that a small mechanical stimulus or solid seeding can trigger a single-crystal-to-single-crystal transformation from a kinetically isolated polymorph of phenyl(phenyl isocyanide)gold(I) exhibiting blue photoluminescence to a thermodynamically stable polymorph exhibiting yellow emission without the need for heating or solvent. The phase transformation initiates at the location of the mechanical stimulation or seed crystal, extends to adjacent crystals, and can be readily monitored visually by the accompanying photoluminescent colour change from blue to yellow. The transformation was characterized using single crystal X-ray analysis. Our results suggest that the transformation proceeds through self-replication, causing the complex to behave as ‘molecular dominoes’.