There is a growing need for technologies that mitigate emissions associated with the continued use of fossil fuels. Carbon capture and storage is a promising large-scale CO₂ mitigation approach. However, the kinetics of mineral dissolution and its impact on solution chemistry and subsequent mineralization potential during early reaction stages remain poorly constrained. This study examines the dissolution kinetics of carbonate rocks under high-pressure brine and brine–CO₂ conditions in the context of geological carbon dioxide (CO₂) storage and mineralization. The primary aim of the study is to quantify the lithology-dependent chemical responses and early-time dissolution behavior under controlled subsurface-like conditions. Experiments were conducted using limestone and dolomite core plugs placed in sealed reaction cells at 2000 psi and 75 °C, with time-resolved measurements of pH and dissolved cation concentrations. Early-time calcium release rates were quantified using concentration–time slopes and were normalized by the geometric surface area to enable direct kinetic comparisons across systems. The results showed systematic differences between lithologies and fluid conditions. Dolomite systems exhibit stronger buffering capacity and higher effective dissolution rates than limestone under brine-only conditions, whereas the presence of CO₂ lowers the initial pH and alters early-time calcium mobilization in a lithology-dependent manner. These trends suggest that the early reaction stages are governed by the kinetics before the onset of progressive pH buffering. Overall, the findings provide quantitative insights into the carbonate dissolution behavior under CO₂-rich conditions and support an improved understanding of mineral–fluid interactions relevant to subsurface CO₂ storage and mineralization strategies.