To address the difficulty of simultaneously achieving wide operating range and high efficiency in LLC resonant converters, as well as the poor dynamic performance and weak disturbance rejection associated with conventional controller-based loop design, an efficiency-optimized control strategy for hybrid-modulated LLC resonant converters is proposed. First, the gain and soft-switching characteristics of the converter under frequency and phase-shift control are analyzed, and a hybrid modulation control scheme is designed. Second, an improved self-disturbance rejection controller based on a low-pass filter is proposed to reduce measurement noise in the extended state observer and enhance the system’s disturbance rejection capability. Then, the losses of each component of the converter are analyzed to construct an efficiency optimization model, and an efficiency optimization method based on the mountain gazelle optimization algorithm is proposed to achieve efficiency maximization under hybrid modulation. By curve fitting of the optimal modulation parameters, the complexity of control loop design is reduced. Finally, an experimental platform is built to validate the theoretical analysis, and experimental results confirm the effectiveness and feasibility of the proposed efficiency optimization method and control strategy.