Background: Being an informal caregiver (IC) of a cancer patient is often associated with psychological distress. We have recently, in a randomized controlled trial (RCT), demonstrated efficacy of Emotion Regulation Therapy for ICs (ERT-C), evidenced as lower levels of psychological distress. Such efficacy demonstration is important, but a crucial step in improving treatments for the IC population is the identification of moderators (i.e., for whom the treatment works) and mediators (i.e., the drivers of the detected effect). Material and methods: In a sample of 65 psychologically distressed ICs (combining participants who received immediate and delayed treatment in the RCT); we investigated age, gender, and homework completion as moderators of treatment outcome. Proposed mediators were derived from the ERT model and included mindfulness, emotion regulation dysfunction, decentering, and cognitive reappraisal. Results and conclusions: The strongest moderation effect was found for homework completion, predicting improvements on psychological distress. Correlational mediation analyses generally supported the ERT model. However, temporal precedence was only established for the association between decentering and worry, where a bidirectional relation was revealed. Homework thus emerged as an important aspect of ERT-C and, albeit a bidirectional relationship, changes in decentering may precede changes in worry. Future trials should ensure the robustness of these results, hone the specificity of process measures, and further investigate the causal timeline of change.