Colliding-pulse mode-locked lasers are an excellent source of ultrashort pulses but exhibit timing jitter that can seriously degrade the temporal resolution in experiments that require synchronization to an external source. We describe a method that synchronizes the colliding-pulse mode-locked laser to a high-stability external oscillator by actively controlling the cavity length of the laser and reduces the absolute timing jitter from ~25 ps rms to only 1.7 ps over the 1-kHz bandwidth, which contains most of the laser phase noise. By using this method, the repetition rate of the laser pulses is held constant at 100 MHz.