With the rapid deployment of 5G and the emergence of 6G technologies, the demand for high-performance microwave dielectric ceramics (MWDCs) has increased. This study developed Y₃MgAl₃GeO₁₂ (YMAG) garnet ceramics to meet 5G/6G requirements for low signal delay, low loss, and high-temperature stability. YMAG ceramics synthesized via a solid-state reaction were characterized for phase composition, crystal structure, microstructure, and microwave dielectric properties. The results revealed that YMAG ceramics exhibited excellent microwave performance: a permittivity (ε_r) of 9.86, a quality factor (Q×f) of 89,000 GHz, and a temperature coefficient of resonant frequency (τ_f) of −40 ppm/°C. Far-infrared and terahertz (THz) spectroscopic analyses verified the low intrinsic dielectric loss and frequency-stable dielectric characteristics of the material in high-frequency ranges. Temperature-dependent dielectric measurements coupled with thermal expansion studies revealed outstanding stability in this material, as evidenced by its low coefficient of thermal expansion (α_L = 9.13 ppm/°C). To attain near-zero τ_f, we added TiO₂ as a positive τ_f compensation agent. This strategy effectively tuned the τ_f value to within |τ_f| < 10 ppm/°C while preserving excellent microwave dielectric performance (Q×f ≈ 43,000 GHz). Furthermore, a rectangular dielectric resonator antenna (DRA) designed with the optimized YMAG–TiO₂ composite demonstrated excellent impedance matching (VSWR = 1.02) and high radiation efficiency (> 90%) in the X-band (10.21 GHz), validating its potential for 5G/6G applications. This work provides a novel approach for developing high-performance MWDCs for next-generation communication technologies and emphasizes the critical role of material design and optimization in achieving superior microwave properties.