Aramid nanofibers (ANFs), typically produced by exfoliating aramid microfibers (Kevlar) in alkaline media, exhibit excellent mechanical properties and have therefore attracted increased attention as nanoscale building blocks. However, the preparation of aramid microfibers involves laborious and hazardous processes, which limits the industrial-scale use of ANFs. This work describes a facile and direct monomer-to-ANF synthesis via an as-synthesized intermediate low-molecular-weight poly(p-phenylene terephthalamide) (PPTA) without requiring the environmentally destructive acids and high-order shearing processes. Under the employed conditions, PPTA immediately dissociates and self-assembles into ANFs within a time period of 15 h, which is much shorter than the time of 180 h (not including the Kevlar preparation time) required for the Kevlar-to-ANF conversion. Interestingly, the fabricated ANFs exhibit nanoscale dimensions and thermoplastic polyurethane (TPU) reinforcing effects similar to those of Kevlar-derived ANFs; i.e., a 1.5-fold TPU toughness improvement and a maximum ultimate tensile strength of 84 MPa are achieved at an ANF content of only 0.04 wt %. Remarkable reinforcement ability investigated by comprehensive analytical data comes from ANFs, which disturb ordered hydrogen bonding in hard segments and induce strain hardening along the elongation pathway. Thus, the developed approach paves the way to industrial-scale production of ANFs and related nanocomposites.