Mode-locking and Q-switching are two widely used pulsing techniques based on different mechanisms. In practice, both lasing regimes can be realized within the same laser system, while the current research on their transition dynamics presents challenges in both numerical calculation and experiments and therefore remains confined only at the observation level. Here, for the first time, the mode-locked and Q-switched regimes are unified under a proposed model, addressing this challenge of simultaneously capturing both gain evolution and nonlinear propagation for both regimes. In light of this model, a robust method based on optical feedback is developed and verified experimentally, demonstrating a convenient real-time switching between the mode-locked and Q-switched states. The measured transitional dynamics closely align with the theoretical model, showcasing the strong correlation between pulse instability and gain depletion that connects the two regimes. The results uplift the significance of the often-overlooked gain dynamics and deepen the understanding of the onset and demise of mode-locked and Q-switched regimes.