Translating the advantages of carbon nanomaterials into macroscopic energy storage devices is challenging because the desirable nanoscale properties often disappear during assembly processes. Here we describe a new nonequilibrium subcritical hydrothermal method capable of independently manipulating the temperature and pressure to create unique assembly conditions crossing the commonly used liquid-vapor boundary. Highly conductive and dense-packed yet ion-accessible nanocarbon microfibers can be obtained from graphene oxide sheets, single-walled carbon nanotubes, and a nitrogen-doping crosslinker under 20 min of hydrothermal assembly, 80% energy saving compared to standard hydrothermal methods, and one of the shortest time in the field of hydrothermal processing of carbon nanomaterials. Using those microfibers, we built microsupercapacitors that reach a high volumetric capacitance of 52 F cm−3, energy density of 7.1 mWh cm−3, and power density of 1645.7 mW cm−3, respectively. We further demonstrate the 3D integration of multiple fiber microsupercapacitors that reduces the device footprint by 75% while expanding the operational voltage and current window. This strategy is a promising tool for harmoniously assembling carbon nanostructures as energy storage components for various energy applications. A new hydrothermal system is capable of controlling the temperature and pressure of water independently to create unique hydrothermal assembly conditions for achieving ultrafast assembly of carbon nanomaterials into carbon architectures. Display omitted