This work established an energy-saving and straightforward treatment of cellulosic pulp to obtain functional cellulose nanofibers equipping them at the same time with catalytic activity and flame-retardant properties. For this purpose, dried cellulose pulp was mixed with a recyclable swelling agent, ZnCl2 hydrate, at room temperature. The mild treatment affected the crystal structure through a partial amorphization, yielding a mix of native cellulose I and regenerated cellulose II. This treatment tremendously facilitated the fibrillation into a cellulose nanofiber (CNF) network. In comparison to fibrillated cellulose from nontreated pulp, the ZnCl2-treated counterpart featured higher viscosity, film transparency, better mechanical properties, and higher heat stability. Films produced from these nanofibers showed flame-retardant properties without any further modification. The ZnCl2-CNF showed also high reactivity in fiber surface acetylation and allowed a fast and efficient reaction while using very mild conditions. All in all, we propose a simple and resource-efficient cellulose treatment to obtain a nanostructured cellulose. These nanofibrils are decorated with ZnCl2 which imposes flame-retardant properties and confined catalytic activity at the fibril surface.