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Ni–Zn–Mg ferrites having the chemical composition Ni0.7-xZnxMg0.3Fe2O4 (x = 0.1, 0.2, 0.3, 0.4, 0.5) were synthesized using easy and low cost-effective sol–gel autocombustion method. Thermogravimetric and differential thermal analysis (TGA–DTA), X-ray diffraction (XRD), Scanning electron microscopy (SEM), and Transmission electron microscopy (TEM) characterization techniques were used to characterize the synthesized samples. Structural and morphological properties of samples calcined at 500 °C, 600 °C, and 700 °C for 8 h were studied from XRD analysis and SEM analysis. Pure cubic phase along with proper grain growth was observed at calcined temperature 700 °C for 8 h. From TEM study of Ni0.4Zn0.3Mg0.3Fe2O4 average grain size is observed to be in range 50–55 nm that revealed the formation of nanosized particles. Thermoelectric power measurement (TEP) shows the p-type behavior at low temperature and n-type behavior at high temperature. p–n transition temperature for Ni–Zn–Mg ferrite system under investigation lies in the temperature range 90–135 °C. Gas sensing properties of Ni–Zn–Mg ferrite thick films toward NO2 and NH3 gases revealed Ni0.6Zn0.1Mg0.3Fe2O4 ferrite thick film shows good response along with quick response and recovery time as compared to other compositions under investigation. [ABSTRACT FROM AUTHOR]