Freeze-thaw cycle (FTC) deterioration poses significant durability challenges to concrete structures. However, the detailed quantitative relationship between FTC-induced internal damage pattern development and mechanical degradation remains underexplored, limiting the accuracy of FTC deterioration assessments. This study aims to provide a comprehensive analysis of microstructural damage progression and its quantitative relationship with mechanical degradation in mortar specimens throughout FTC process. Through establishing an experimental setup applied to specimens at two scales and facilitated by micro-computed tomography (micro-CT), this research concurrently evaluates the microstructural damage progression, mechanical degradation, and expansion of the specimens under various FTC severity levels. The quantification of multiple crack indices reveals a stronger correlation between compressive strength reduction and crack ingression than with increases in crack quantity or width. The findings further highlight the imperative to incorporate both expansion-induced damage and time-dependent fatigue damage in evaluating FTC deterioration, thus laying a foundation for refining FTC prediction models.