The historical narrative of increasing chromaticism and dissonance in Western music, while widely accepted, has predominantly relied on traditional close-reading analyses of individual works rather than comprehensive empirical validation. This study presents a computational analysis of a large-scale dataset—comprising over 230,000 expert-annotated harmonic labels from 1280 musical works spanning three centuries—to quantify and trace the diachronic development and co-evolution of chromaticism and dissonance. We introduce novel quantitative indices measuring chromaticism and dissonance to evaluate stylistic evolution in different musical periods. Our findings empirically validate the established narrative of a gradual rise in both chromaticism and dissonance, and further reveal a more nuanced trajectory characterized by modal differences and a persistent diatonic foundation alongside chromatic and dissonant experimentation. Adopting a “distant listening” approach, this study provides a systematic and quantitative perspective on the evolution of chromaticism and dissonance, complementing traditional musicological approaches and providing novel, data-driven insights into the evolution of Western tonal music.