Fabrication and characterization of fiber optic temperature sensors using Al2O3-MgO nanocomposite as cladding material have been reported. Co-precipitation route was adopted to synthesize Al2O3, MgO and their composites of different composition. The synthesized Al2O3-MgO nanocomposites (25%-75%, 50%-50% and 75%-25%) of different mole concentrations have been characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy and UV-Visible spectroscopy to investigate the structural, morphological, elemental and optical properties. The temperature sensing probe was fabricated by coating Al2O3-MgO nanocomposites over the unclad portion of an optical fiber. The response of the sensors has been studied in the temperature range of 35 °C-80 °C. Further, the experimental result depicts an exorbitant temperature sensitivity of ∼10.8 towards Al2O3-MgO nanocomposites (50%-50%) than other composites, pristine Al2O3 and MgO. In addition the sensor exhibits an ultrahigh sensitivity of 0.62%/°C with a better linear regression coefficient of 95%. Therefore, the proposed fiber optic sensor is capable of measuring dynamic temperatures in relentless environment. © 2018 IOP Publishing Ltd.