Phosphorylated cellulose nanofibrils (CNFs) were produced using a twin-screw extruder (TSE). The energy efficiency was compared with that of an ultra-fine grinder (UFG). CNFs at different solid contents were produced (10 wt% for the TSE and 2 wt% for the UFG), and their quality was compared using several characterization tests, such as optical microscopy, atomic force microscopy or MorFi fiber analyser. CNF nanopapers with high a Young’s modulus (14 GPa) and high transparency were successfully manufactured using CNFs produced by the TSE. The efficiency of the TSE was improved by using phosphorylated pulp instead of mechano-enzymatically treated pulp. The results show that phosphorylated CNFs can be easily produced at the industrial scale via twin-screw extrusion and present new functionalities such as fire-retardant properties, which are useful in many applications.