Magnetic field sensors have been widely applied in several areas, for instance, in navigation, geophysical, aerospace engineering and biomedical research. The traditional methods used to sense this parameter have drawbacks related with size, stability, multiplexing capability, remote measurement and electromagnetic sensitivity. Due to the characteristics inherent to the optical fiber, including small dimensions, immunity to electromagnetic interference and the possibility of being used in hazardous environments, this technology has great potential for sensing different parameters. In this work, the magnetic field was monitored using a Fabry-Perot micro-cavity. The cavity, produced from the recycling of optical fiber previously destroyed by the catastrophic fuse effect, was filled with magnetic fluid (MF). Then, it was exposed to a magnetic field in the range of 0 to 200 mT, applied transversally to the fiber axis. An overall exponential decrease of the wavelength of the reflection spectrum with the increase of the magnetic field was obtained, with a sensitivity and resolution of 120.5 ± 4.4 pm/mT and 8.3 μT, respectively, in the range of 0 to 80 mT (linear behavior). The proposed sensor represents a cost-effective solution for the magnetic field sensing, with an improved performance compared with other devices already reported in the literature.