In this study, a novel spinel solid solution ceramic of 0.4LiFe5O8–0.6Li2MgTi3O8 (0.4LFO–0.6LMT) has been developed and investigated. It is found that the 40 mol% LiFe5O8 and 60 mol% Li2MgTi3O8 are fully soluble in each other and a disordered spinel phase is formed. The ceramic sample sintered at 1050°C/2 h exhibits both good magnetic and dielectric properties in the frequency range 1–10 MHz, with a permeability between 29.9~14.1 and magnetic loss tangent between 0.12~0.67, permittivity between 16.92~16.94 and dielectric loss tangent between 5.9 × 10−3–2.3 × 10−2. The sample also has good microwave dielectric properties with a relative permittivity of 16.1, a high quality factor (Q × f) ~28 500 GHz (at 7.8 GHz). Furthermore, 3 wt% H3BO3–CuO (BCu) addition can effectively lower the sintering temperature to 925°C and does not degrade the magnetodielectric properties. The chemical compatibility with silver electrode indicates that this kind of ceramics is a good candidate for the low-temperature cofired ceramic (LTCC) application.