To achieve subpicometer sensitivities in the millihertz band, laser interferometric inertial sensors rely on some form of reduction of the laser-frequency noise, typically by locking the laser to a stable frequency reference, such as the narrow-line-width resonance of an ultrastable optical cavity or an atomic or molecular transition. In this paper, we report on a compact laser-frequency stabilization technique based on an unequal-arm Mach-Zehnder interferometer that is subnanometer stable at 10μHz, subpicometer at 0.5 mHz, and reaches a noise floor of 7fm/Hz at 1 Hz. The interferometer is used in conjunction with a dc servo to stabilize the frequency of a laser down to a fractional instability below 4×10-13 at averaging times from 0.1 to 100 s. The technique offers a wide operating range, does not rely on complex lock-acquisition procedures, and can be readily integrated as part of the optical bench in future gravity missions.