Due to its excellent magnetic properties, MgZn ferrite is a nearly optimal material for low temperature co-fired ceramic (LTCC) phase shifter applications. In this study, low temperature co-fired Mg0.8Zn0.2-xCdxFe2O4 ferrites (x = 0.00–0.10, with increments of 0.02) were synthesized using a solid-state method with the aid of 2.5 wt% BBSZ (33%mol Bi2O3-21%mol B2O3-11%mol ZnO-35%mol SiO2) glass sintering. The effects of the Cd2+ ions on phase formation, microstructure, magnetic permeability, and gyromagnetic properties were investigated. The results indicated that a suitable amount of Cd2+ ion substitution did not change the phase formation of MgZn ferrite. However, with further increases of the Cd2+ ion content, the superfluous Cd2+ ions and Fe3+ ions formed CdFe2O4, which affected the microstructure, density and magnetic properties of the samples. More specifically, when x = 0.04, Mg0.8Zn0.16Cd0.04Fe2O4 ferrite, sintered at 920 °C, showed excellent magnetic permeability (μ'~56.6 @1 MHz–20 MHz) and had a high cut-off frequency (~100 MHz). Furthermore, the ferromagnetic resonance line widths (ΔH) were measured as a function of the Cd2+ substitution at 9.56 GHz, and the relationships between ion occupancy and microstructure were also discussed. A narrow ΔH (~228.2 Oe) can be obtained by adding an optimal amount of Cd2+ ions. These observations indicate that MgZn ferrites with a suitable amount of Cd2+ ions are promising candidate materials for LTCC electronic device applications.