Crossbred photocatalysts with dielectric and magnetic are crucial by its vantage of being convenient in sundry applications. In this paper, lanthanum with large percentages doped zinc magnesium ferrite (Zn0.8Mg0.2LaxFe2-xO4) (ZML) nanoparticles with x ranging from 0.0 to 0.5 prepared via a combustion method. The spinel structure of (ZML) nanoparticles is asserted, and with further La3+ substitution, the lattice parameter is found to increase from 8.379 to 8.433Å with a monotonic shift in diffraction peak (311) toward lower degree angles (2θ= 35.28ο to 34.0o). From Williamson-Hall plots, the crystallite size has an increasing demeanor (28-49 nm) and lattice strain declaring the increasing tensile strain. FE-SEM depicts flakes with different size and shape with porous nature. EDX spectra divulge the entity of all chemical elements. HRTEM and SAED micrographs confirmed the polycrystalline nanosized essence of samples. The finger print ferrite FTIR absorption bands were obtained. Crystallite size role and the magnetocrystalline anisotropy property of La3+ are the substantial reasons for the demeanor of peculiar coercivity. The boosting attitude of dielectric constant as well as conductivity and diminishing of tangent loss are based on the effective role of porosity and crystallite size. Tauc's plots introduced direct allowed Eg for ZML nanoparticles with a red shift from (1.92eV at x= 0.0 to 1.66 eV at x= 0.5); owing to three factors. The photodegradation percentage of RhB dye in the presence of the nanoferrite Zn0.8Mg0.2La0.5Fe1.5O4 is increased drastically with the irradiation time increment from (17% at t= 30min to 84% at t= 270min). The optimum features of the nanoferrite Zn0.8Mg0.2La0.5Fe1.5O4; high coercivity, dielectric constant, conductivity, photodegradation percentage, besides low loss and energy gap make it advisable for various applications as storage and high frequency devices and photocatalyst for disposing effluents from water.