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Insight into the aggregation kinetics of gold nanoparticles (GNPs) is critical for developing a colorimetric assay extensively used in chemical and biomolecular sensing. The aggregation of NPs plays a significant role in many natural and industrial processes, demanding comprehensive perceptions of the aggregation kinetics at a solid–liquid interface. However, the direct observation of the melamine-induced aggregation process of GNPs in the time-domain still remains a challenge. There is little to no information on the fundamental mechanisms of such kinetics using evanescent waves. Total internal reflection (TIR) has been applied to generate the evanescent field (EF), exploring aggregation kinetics near the solid–liquid interface. Here, we utilized a precise optical cavity-based method, an evanescent-wave coupled cavity ring-down spectroscopy (EW-CRDS), that can probe the melamine-induced aggregation kinetics of GNPs. The key feature of the present method is that the evanescent field generated by TIR illumination harnesses the power of CRDS to study 2D fractals via the collision and attachment of the GNPs and their melamine-induced aggregates at the interfacial region in real-time. This kinetic study reveals a critical point for diffusion-limited aggregation and provides insights into the design and optimization of colorimetric sensors that exploit the aggregation of GNPs. Furthermore, the EW-CRDS is a unique analytical approach that helps to deepen our understanding in probing the real-time aggregation process, detecting the presence of aggregator as compared to UV–vis and dynamic light scattering (DLS) spectroscopy.