Ni (x) Zn0.9-x Mn0.1Fe2O4 (0.3 a parts per thousand currency signxa parts per thousand currency sign 0.7) ferrites were synthesized using the conventional ceramic processing method. A 1 wt% B2O3 was added to initial mixture to inhibit crystal growth and to decrease phase formation temperature. The structural, magnetic, dielectric, and microwave properties of samples have been investigated using X-ray diffraction (XRD), scanning electron microscope (SEM), vibrating sample magnetometer (VSM), and near-field microwave measurements in RF Impedance Analyzer. Magnetization measurements revealed that saturation magnetization of the Mn-substituted Ni-Zn ferrite increases as nickel concentration increases up to x= 0.6. The saturation magnetization has the highest value of 78 emu/g in the Ni0.6Zn0.3Mn0.1Fe2O4 sample. All samples have negligible coercive fields of few oersteds. Microwave measurements revealed that ferromagnetic resonance occurs at around 1.8 GHz and minimum reflection loss attained is less than -20 dB with sample thickness of 10 mm. The observed resonance is due to the domain wall motion. As a result, these materials can be used in microwave absorbers in low-frequency range (i.e., 0.1-3 GHz) as magnetic fillers.