Light-shining-through-a-wall experiments represent a new experimental approach to search for undiscovered elementary particles not accessible with accelerator based experiments. The next generation of these experiments, such as ALPS II, require high finesse, long baseline optical cavities with fast length control. In this paper we report on a length stabilization control loop used to keep a cavity resonant with light at a wavelength of 532nm. It achieves a unity-gain-frequency of 4kHz and actuates on a mirror with a diameter of 50.8mm. This length control system was implemented on a 10m cavity and its projected performance meets the ALPS II requirements. The finesse of this cavity was measured to be 93,800\(\pm\)500 for 1064nm light, a value which is close to the design requirements for the ALPS II regeneration cavity.