The relationship between affect and creative performance in a practical group project environment is poorly understood. The role of leadership in creativity is also often neglected. The present study examines how the affective aspects of group members and emergent leadership influence the creative group performance of tasks. This empirical research engaged the participation of 17 graduate students of a graduate course on design thinking in an engineering school of a university. Data were collected over the course of five class meetings during which the participating students accomplished the performance of seven tasks and shared their perceptions with regard to the affect and leadership aspects of the achievement of the assigned work. The results showed coherence between the performance of diverse tasks and the emotions of participants, especially in terms of the psychological state of arousal. Meanwhile, task-oriented leadership was found to be positively correlated with valence in the storytelling task and with performance in most of the convergent tasks. This study accords a deeper understanding of the ways in which creative group work differs from individual creative effort in relation to affects. It also reveals the importance of leadership in creative group work.