The hybridization of highly porous carbon materials and battery-type materials is an effective method for achieving supercapacitors with both high energy density and good rate capability. The structure of the carbon substrate should be carefully designed to meet the requirement of good conductivity and good accessibility to electrolyte and ions. In this work, graphene nanoribbon crosslinked polyimide-derived carbon aerogels (a-GCA) with hierarchical porous structures and large specific surface area of 2413.0 m2 g−1 have been first constructed, and then applied as a conductive template for the uniform growth of ultrathin Ni(OH)2 nanosheets. In the obtained Ni(OH)2/a-GCA hybrids, a-GCA can provide open and interconnected channels for rapid diffusion of ions and electrons to access Ni(OH)2 nanosheets for fast Faradaic redox reactions, as well as enhance the stability of Ni(OH)2 nanosheets. Consequently, the optimized Ni(OH)2/a-GCA hybrid exhibits a high specific capacitance of 537.0 C g−1 at a charge/discharge current density of 1 A g−1, excellent rate capacitance retention of 78.4% at 10 A g−1. Moreover, the assembled Ni(OH)2/a-GCA//a-GCA hybrid supercapacitor device delivers a high energy density of 54.8 Wh kg−1 at a powder density of 816 W kg−1. Therefore, the Ni(OH)2/a-GCA hybrid shows great potential as high-performance electrode materials in applications in energy storage device.