Via a facile ultrasound synthesis from nickel acetate and sodium hydroxide with ionic liquids as the solvent and template it is possible to obtain nano-β-Ni(OH)2 of various dimensionalities depending on the reaction conditions with the ionic liquid (IL) being the most important factor. Scanning electron microscopy (SEM) imaging showed β-Ni(OH)2 to form as nanosheets, nanorods and nanospheres depending on the IL. ILs with strong to moderate hydrogen bonding capability like [C3mimOH][Tf2N] (1-(3-hydroxypropyl)-3-methylimidazolium bis(trifluoromethanesulfonylamide)), [C4mim][Tf2N] (1-butyl-3-methylimidazolium bis(trifluoromethanesulfonylamide)) and [Edimim][Tf2N] (1-ethyl-2,3-diemethylimidazolium bis(trifluoromethanesulfonylamide)) lead to the formation of nanosheets whilst [Py4][Tf2N] (butyl-pyridinium bis(trifluoromethanesulfonylamide)) leads to nanoparticles and [N1888][Tf2N] (methyltrioctylammonium bis(trifluoromethanesulfonylamide)) to nanorods. Subsequent calcination of the materials at elevated temperatures (285–425 °C) leads to the conversion of β-Ni(OH)2 to NiO under preservation of the nanostructure. Scanning electron microscopy (SEM), X-ray diffraction (XRD), TG-DTA, X-ray photoelectron spectroscopy (XPS), and energy dispersive X-ray spectroscopy (EDX) were used to observe the morphology, crystallinity, and chemical composition in more detail. Mesoporous NiO nanosheets obtained in [C4mim][Tf2N] possess an exceptionally high surface area of 141.28 m2 g−1 and a pore volume of 0.2 cm3 g−1 at 285 °C. As a result of calcination at 425 °C the surface area decreased to 92.84 m2 g−1, but the pore volume increased to 0.48 cm3 g−1. In addition, the product has an extraordinarily high saturation magnetization of 1.38 emu g−1, a coercivity of 117 Oe and an excellent specific capacitance of 199.4 F g−1 which renders the material highly interesting for application in supercapacitors.