The development of a reliable technology for domain engineering in thin film lithium niobate is crucial to leveraging its disruptive potential for integrated nonlinear optics. However, thin film formats present outstanding challenges for traditional poling techniques with specific concern to non-polar cuts and short periods. Here, a novel approach is developed for the periodic poling of x-cut ≈500nm-thick lithium niobate on insulator (LNOI), relying on electron beams. High-quality ferroelectric gratings with periods in the 3.5–0.37 µm range are successfully fabricated, and a comprehensive analysis of their properties by piezoresponse force microscopy is presented, providing evidence for poling in highly non-equilibrium regimes, yielding regular domain gratings that remain stable over several years. Moreover, seamless integration with undoped and 5 mol% MgO-doped LNOI photonic nanowires is demonstrated, together with their nonlinear optical functionality in both co- and counter-propagating waveguide experiments. This novel poling technology appears ideally suited for submicrometric domain patterning of the most widely used cut for LNOI photonic integrated circuits and holds promise for unlocking their full potential for the realization of ultralow-footprint all-optical signal processing chips exploiting engineerable nonlinearities for classical and quantum applications.