Composition and structure play dominant roles in realizing the microwave dielectric properties that are necessary for the ever-increasing demands of the Internet of Things and related communication technologies. In the present study, the substitution of Ta5+ in Li3Mg2Nb1-xTaxO6 ceramics and its effect on the structural characteristics and microwave dielectric performances is systematically studied. All the substituted compositions were determined to be pure phase orthorhombic Li3Mg2NbO6 structure of space group Fddd. Furthermore, a NbO6 octahedral distortion, Nb-O bond valence, packing fraction and polarizability were calculated to explore the structure-property-performance paradigm in the context of microwave dielectric performance. Scanning electron microscopy revealed homogeneous microstructures, with the introduction of Ta5+ promoting grain growth. Raman spectra indicated that the variation of the band (blue shift and red shift) at 771 cm(-1) was highly correlated with the variation in unit cell volume. The polarizability significantly impacted e(r) values. The Q x f values were strongly influenced by the packing fraction and grain size. The changes in the NbO6 octahedral distortion and Nb-O bond valence impacted the tau(f) values. The Li3Mg2Nb0.98Ta0.02O6 composition displayed the most dramatic improvements in microwave dielectric properties: epsilon(r) = 15.58, Q x f = 113 000 GHz and tau(f) = -4.5 ppm/degrees C, providing a potential candidate for next generation microwave and millimeter-wave applications.