Shale hydration and swelling are disadvantageous for well drilling, especially when using water-based drilling fluids. In this work, the ionic liquid 1-vinyl-3-ethylimidazolium bromide (VeiBr) monomer and its corresponding homopolymers (PV) were innovatively used as shale hydration inhibitors. Both composites of sodium bentonite (Na-BT) with VeiBr and PV (hereafter denoted as Na-BT/VeiBr and Na-BT/PV composites) exhibited excellent temperature stability up to 300 °C, showing potential application in high-temperature well drilling. The inhibiting performance was evaluated by measuring the linear swelling height, rheological property of Na-BT aqueous solutions, and recovery percentage of shale cuttings after hot rolling. Results indicated that VeiBr monomer and PV polymer displayed better inhibition performance than inorganic KCl and organic quaternary amine 2,3-epoxypropyltrimethylammonium chloride in all tests. In addition, PV was even better than VeiBr. The underlying mechanism was analyzed by measuring the interlayer distance through X-ray diffraction, observing the aggregation through scanning electron microscopy, and determining the ζ potential and particle size distribution. The monomer exerted its effect mainly by decreasing the interlayer spacing, whereas the polymer increased the viscosity, encapsulated Na-BT particles, prevented the exfoliation of Na-BT, and decreased the interlayer spacing depending upon the molecular weight. This study can serve as a basis for using ionic liquids in the design of permanent shale inhibitors for drilling fluids.