The residual life of metallic structures is dictated by microstructural plastic damage, which conventional linear ultrasonics often fail to detect early. This study investigates the continuous, in-situ evolution of the second-order acoustic nonlinearity parameter (β₂) throughout the monotonic tensile response of S235 structural steel, from the elastic regime to fracture. Bulk longitudinal waves were excited by a contact piezoelectric transducer and detected through-thickness using a non-contact Laser Doppler Vibrometer on a notched specimen. This allowed the normalized acoustic parameters to be mapped directly onto the macroscopic stress-strain curve. Three distinct acoustic regimes were identified. Up to the onset of the yield plateau, the nonlinearity parameter and the linear attenuation remained at unity within the specimen-to-specimen scatter. Beyond the yield point, β₂/β₂₍₀₎ rose steeply to approximately 4.5, whereas the normalised attenuation increased by at most approximately 20%. The rise extended over the yield plateau of the stress–strain curves and the parameter saturated when work hardening resumed (nominal strain ≈ 0.031–0.033), ahead of the maximum load (≈0.043); a lower value was recorded only at the last acquisition, at which the signal quality was degraded. The departure of β₂ from unity, i.e., the first increase of the second-harmonic amplitude, occurred between nominal strains of 0.015 and 0.021, coinciding with the onset of the yield plateau (≈0.017) and slightly above the 0.2% offset yield strain (0.0128 ± 0.0004), as expected for a notched specimen in which plasticity reaches the interrogated volume once it spreads across the ligament; this supports its potential as a marker of yield onset. The increasing trend provides a continuously acquired ferritic-pearlitic reference data set and is consistent with, although not by itself proof of, a governing role of the initial dislocation substructure in harmonic generation.