Magnetic fields play an important role in the evolution of interstellar medium and star formation. As the only direct tracer of interstellar field strength, credible Zeeman measurements remain sparse due to rather limited number of spectral lines with discernible Zeeman effect, particularly for cold, molecular gas. Here we report the detection of a magnetic field of +3.8 ± 0.3 μG through the HI narrow self-absorption (HINSA) toward the prestellar core L1544. A combined analysis of the Zeeman measurements of quasar HI absorption, HI emission, OH emission, and HINSA reveals a coherent magnetic field from the atomic cold neutral medium (CNM) to the molecular envelope of the L1544. We find that the molecular envelope traced by HINSA is already magnetically supercritical, with a field strength comparable to that in the surrounding diffuse, magnetically subcritical CNM despite a large increase in density. The reduction of the magnetic flux relative to the mass, necessary for star formation, thus seems to happen during the transition from the diffuse CNM to the molecular gas traced by HINSA, earlier than envisioned in the classical picture where magnetically supercritical cores capable of collapsing into stars form out of magnetically subcritical envelopes. (Nature Accepted)