We present a portable optically pumped magnetometer instrument for ultrasensitive measurements within the Earth’s magnetic field. The central part of the system is a sensor head operating a MEMSbased Cs vapor cell in the light-shift dispersed Mz mode. It is connected to a compact, battery-driven electronics module by a flexible cable. We briefly review the working principles of the device and detail the realization of both the sensor head and electronics. We show shielded and unshielded measurements within a static magnetic field amplitude of 50 μT, demonstrating a noise level of the sensor system down to 140 fT/√ Hz and a sensor bandwidth of several 100 Hz. In a detailed analysis of sensor noise, we reveal the system to be limited by technical sources with straightforward strategies for further improvement toward its fundamental noise limit of 12 fT/√ Hz. We assess the parameters defining the sensor bandwidth by theoretical modeling based on the Bloch equations. Finally, we compare our sensors’ performance to a commercial superconducting quantum interference device system in a measurement environment typical for geomagnetic observatory practice and geomagnetic prospection.