Spin-glass theory has been widely introduced to describe the statistical behaviors in complex physical systems. By analogy between disorder photonics and other complex systems, the glassy behavior, especially the replica symmetry breaking (RSB) phenomenon, has been observed in random lasers. However, previous studies only analyzed the statistical properties of the random laser systems with single gain material. Here, the first experimental evidence of the glassy behavior in a random laser with complex energy level structure is reported. This novel random laser is demonstrated based on the electrospun polymer fibers with the assistance of Förster resonance energy transfer (FRET). The electrospinning technology employed in the experiment herein promises high-volume production of random laser devices with multiple energy levels, enabling the comprehensive investigation of lasing properties in multi-energy level random laser system. Clear paramagnetic phase and spin-glass phase are observed in the FRET-assisted random laser under different pump energies. The RSB phase transition is verified to occur at the laser threshold, which is robust among the random lasers with different donor–acceptor ratio. The finding of RSB in FRET-assisted random laser provides a new statistical analysis method toward the laser system with complex energy level, for example, quantum cascade laser.