Reliable formation evaluation using borehole geophysical measurements in organic-rich mudrocks requires knowledge about the physical properties of kerogen. For instance, estimates of water-filled pore volume are significantly affected by the assumptions made for dielectric permittivity of kerogen, which can be influenced by thermal maturity. However, the impact of thermal maturity of pure kerogen on its dielectric properties is not yet thoroughly understood. We quantify the dielectric constant of kerogen samples extracted from three formations, with different levels of natural thermal maturity, and we identify the impact of thermal maturity on their dielectric properties. We first isolate kerogen from mudrock samples using physical and chemical treatments. We then synthetically mature the samples in a controlled environment and measure their dielectric constant (at 1 GHz) using a microwave resonator. X-ray photoelectron spectroscopy (XPS) monitors the variation in chemical composition of kerogen. The dielectric constant of the kerogen samples varied significantly in the range of 1.89–3.2 upon being heat treated from 25°C to 650°C. The variation in the dielectric constant is explained by the alteration in the chemical composition and structure of kerogen as a result of thermal maturation. XPS measurements also showed an increase in aromatic carbon content in the kerogen samples as the thermal maturity increased. The documented results enable the integration of the kerogen geochemistry to the interpretation of dielectric measurements, which contributes to improved interpretation of dielectric logs in organic-rich mudrocks, and result in enhanced formation evaluation of these reservoirs.