A magnetostrictive fiber optic interferometric sensor that detects low-intensity magnetic fields has been realized. The sensor is fabricated with 850-nm, single-mode fiber optic cable and couplers configured in a Mach-Zehnder interferometric arrangements. Four sensing arm configurations were fabricated where the magnetostrictive material was geometrically coupled to the fiber optic cable as a: ribbon, cylinder, sandwich, and sputter-coated sheath. The four classes of sensing arms were evaluated using MetglasTM, nickel, and a combination of MetglasTM and nickel. Optimization techniques were applied to maximize the sensor's sensitivity. These techniques included: annealing the magnetostrictive material, operating the magnetic field over a range of frequencies, applying a DC bias current to the Helmholtz coil's AC current component, and fabricating sensing arms with polarization maintaining fiber optic cable. The measurements employing the various sensor arm classes revealed that the lowest detectable magnetic flux density was 0.3 mG. This result was achieved using a MetglasTM ribbon-fiber optic cable configuration where the ribbon's magnetization direction was aligned perpendicularly with the applied 52 kHz AC magnetic field that had a 5- volt DC bias. When operating this configuration with a 10 Hz AC magnetic field that had a 5- volt DC bias, the lowest magnetic flux density that could be detected was 8.9 mG.