Structural, magnetic, and electrical characteristics of manganites in the system La3+0.7Sr2+0.3Mn3+0.6–x-2γMn4+0.3+x+2γFe3+0.1–x Mg2+xO2-3+γ (0 ≤ x ≤ 0.1) synthesized by solid-state reactions are presented. Sintering was performed in air at 1423 K. Samples with stoichiometric oxygen content (γ = 0) were obtained by annealing at 1223 K and a partial oxygen pressure of 0.1 Pa. All studied manganites have a rhombohedral structure. With an increase in the magnesium content, the volume of the unit cell decreases, and annealed samples have a larger cell volume than the initial, sintered manganites containing over-stoichiometric oxygen. The ratio c/a of lattice parameters is practically unchanged. Curie point of manganites decreases in general with an increase in the magnesium content, showing a weakly expressed plateau in the region of 0.025 < x < 0.075. Magnetization and “metal-semiconductor” transition temperature have maximum values at x = 0, then change non-monotonically, and at x > 0.075 fall sharply. Manganite, which contains the largest amount of magnesium, has the highest resistance at temperatures below 180 K and is characterized by the largest width of the temperature interval of “ferromagnetic-paramagnetic” transition, which indicates the existence of magnetic inhomogeneities. The electromagnetic parameters of the initial and annealed samples differ slightly. A number of effects and competing factors that determine complicated dependencies of electromagnetic characteristics of the manganites of this system on the composition are considered.