Abstract: Distributed roughness elements form on high-speed vehicle surfaces due to material properties, manufacturing techniques, and ablation effects. This surface morphology modifies the boundary layer transition process, which detrimentally impacts the thermal protection system and aerodynamic performance. To elucidate the underlying mechanism through which distributed roughness affects high-speed boundary layer transition and second-mode wave evolution, experimental investigations were performed on a 7° half-angle sharp cone model within the Φ = 0.6 m low-noise wind tunnel (SKLA-TT1). The study employed high-frequency pressure sensors, infrared thermography, Focused Laser Differential Interferometry (FLDI), and Particle Image Velocimetry (PIV). Transition characteristics were analyzed under Mach 6 conditions at zero angle of attack for a smooth cone surface and for surfaces with two configurations of distributed roughness. The roughness heights were set at h = 0.25δ and h = 0.4δ, where δ is the local boundary layer thickness at the axial location of the roughness end in the smooth-wall reference case. Infrared thermography results indicate that the introduction of distributed roughness causes the transition onset to shift upstream. This forward shift is more pronounced for the taller roughness case (h = 0.4δ). Both roughness configurations generate distinct streamwise-alternating bands of high and low surface temperature. Measurements from high-frequency pressure sensors and FLDI demonstrate that the presence of roughness reduces the characteristic frequency of dominant second-mode waves and narrows the downstream bandwidth of amplified unstable frequencies. Spectral analysis of the FLDI data further reveals, for the h = 0.4δ case, a non-monotonic streamwise evolution of the second-mode wave amplitude within the roughness-to-smooth-wall recovery region. The amplitude exhibits an initial growth, followed by decay, and subsequent regrowth. PIV flow field analysis clarifies this behavior: the taller roughness elements induce a localized separation zone and an associated shear layer at their trailing edge. This shear layer attenuates the second-mode waves, leading to the observed amplitude decay. Further downstream, as the separation zone dissipates, the stabilizing effect ceases, allowing the second-mode waves to resume their growth and resulting in the final amplitude increase.