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Mood instability (MI) and functional impairment are core features of Major Depressive Disorder (MDD), yet their dynamic relationship in daily life remains understudied. This pilot study used Ecological Momentary Assessment (EMA) to examine the association between MI and daily functioning in 27 inpatients with MDD and 37 controls. Participants completed baseline assessments of depression severity, daily function, and Quality of Life (QoL), followed by 11 days of EMA monitoring for mood and activities. We calculated MI using the root mean square of successive differences and quantified the uniformity of daily activities using the Gini impurity score. Results indicated that participants with MDD experienced significantly higher MI compared to controls (p < .001). Higher MI was correlated with greater depression severity (p = .022) and lower QoL (p = .022). Furthermore, the MDD group showed lower occupational balance, reduced participation in daily activities (p < .001), and a less uniform distribution of activities (p = .002). Within the MDD group, we observed a complex pattern: higher MI was associated with greater participation levels (measured by the Activity Card Sort; p = .060) and a significantly more uniform activity distribution (p = .022). This suggests that in patients with MDD, greater mood instability may be associated with engagement in a wider variety of occupations. This study presents a novel EMA-based measure for daily function patterns, revealing a complex interplay with mood instability that may reflect a broader imbalance mechanism across multiple domains. These findings underscore the importance of incorporating real-time mood and functional monitoring into clinical assessment and treatment.