Membranes based on complex solid oxides with oxygen-ionic conductivity are widely used in high-temperature electrochemical devices such as fuel cells, electrolyzers, sensors, gas purifiers, etc. The performance of these devices depends on the oxygen-ionic conductivity value of the membrane. Highly conductive complex oxides with the overall composition of (La,Sr)(Ga,Mg)O3have regained the attention of researchers in recent years due to the progress in the development of electrochemical devices with symmetrical electrodes. In this research, we studied how the introduction of iron cations into the gallium sublattice in (La,Sr)(Ga,Mg)O3affects the fundamental properties of the oxides and the electrochemical performance of cells based on (La,Sr)(Ga,Fe,Mg)O3. It was found that the introduction of iron leads to an increase in the electrical conductivity and thermal expansion in an oxidizing atmosphere, while no such behavior was observed in a wet hydrogen atmosphere. The introduction of iron into a (La,Sr)(Ga,Mg)O3electrolyte leads to an increase in the electrochemical activity of Sr2Fe1.5Mo0.5O6−δelectrodes in contact with the electrolyte. Fuel cell studies have shown that, in the case of a 550 µm-thick Fe-doped (La,Sr)(Ga,Mg)O3supporting electrolyte (Fe content 10 mol.%) and symmetrical Sr2Fe1.5Mo0.5O6−δelectrodes, the cell exhibits a power density of more than 600 mW/cm2at 800 °C.