Thin films of ferric oxide (${\alpha}$-Fe$_2$O$_3$) doped with manganese were synthesized on fluorine-doped tin oxide (FTO) glass substrates using the doctor blade method. Iron acetate powder was annealed at 600°C to obtain the crystalline ${\alpha}$-Fe$_2$O$_3$. Polyethylene glycol was used as the binder. Manganese mass concentration in ${\alpha}$-Fe$_2$O$_3$ was changed from 3 to 10% in the doping process. All the thin film samples were subsequently annealed at 120°C for 2 h in air. Samples were characterized using X-ray diffraction (XRD), UV–visible spectroscopy, Fourier transform infrared (FTIR) and X-ray fluorescence (XRF). According to the XRD patterns, single phase of ${\alpha}$-Fe$_2$O$_3$ was crystallized in all the samples. XRD patterns confirmed that the hematite phase of the samples does not vary with doping concentration. FTIR spectra confirmed the formation of the iron oxide bonds without any impurity phases. According to the UV–visible spectroscopy, the lowest bandgap of 1.82 eV could be obtained for the samples with 6% of manganese. XRF analysis was employed to confirm the doping concentration after synthesizing thin film samples. The gas sensitivity of pure ${\alpha}$-Fe$_2$O$_3$ thin films was measured in 1000 ppm of acetone vapour, CO$_2$ gas, ethanol vapour and ammonia gas by means of a Keithley 6400 sourcemeter AUTOLB at the room temperature. For pure ${\alpha}$-Fe$_2$O$_3$ thin films, higher gas sensitivities of 46 and 49.2% were observed for CO$_2$ gas and acetone vapour, respectively. The gas sensitivity of the doped ${\alpha}$-Fe$_2$O$_3$ thin films was measured only in 1000 ppm CO$_2$ gas. The ${\alpha}$-Fe$_2$O$_3$ thin film doped with 6% manganese exhibited remarkable sensing performance of 70.1% at room temperature in CO$_2$ gas.