The locking bandwidth of an actively mode-locked semiconductor laser is a measure of its tolerance to variations in the input drive frequency. At frequencies outside the locking bandwidth, the output pulses from the laser exhibit large amplitude fluctuations and timing jitter. This paper investigates the locking bandwidths of fundamentally driven and harmonically driven high-repetition-rate actively mode-locked semiconductor lasers. We show that the locking bandwidth is maximized when the cavity length is minimized. The locking bandwidth is related to an important constant, the “pull-in time”. Experimental data and numerical modeling show that the pull-in time is a function of the optical bandwidth of the system and the RF drive level.