This study explores the incorporation of zinc–nickel hexaferrite (Zn–Ni ferrite) powders into PLA-based filaments for Fused Filament Fabrication (FFF) 3D printing to enhance magnetic properties. Zn–Ni ferrite powders were mixed with PLA at concentrations of 5, 10, and 15%, and composite granules were produced using a twin-screw extruder. These granules were then used to fabricate filaments with a diameter of 1.75 mm through a single-screw extruder. Mechanical properties were assessed via tensile testing, which revealed that tensile strength was highest with 5% Zn–Ni ferrite but decreased with higher concentrations, potentially due to issues with additive dispersion and bonding. Magnetic properties were evaluated using Vibrating Sample Magnetometry (VSM), which showed that coercivity and permeability increased linearly with additive content, ranging from 8.09 Oe to 9.79 Oe and from 1.06 to 1.08, respectively. These findings demonstrate that Zn–Ni ferrite significantly enhances the magnetic and mechanical properties of PLA filaments, making them suitable for applications in magnetic sensors and electrical devices. The study suggests avenues for future research, including the use of different magnetic powders, exploration of high-temperature resistant matrix materials, and development of magnetic powder-enhanced resins for other additive manufacturing technologies.