Intersubject electroencephalogram (EEG) coherence of 11 couples was measured during an individual and joint practice of a guided mindful breathing exercise. Additionally, the relationship of personality dimensions of agreeableness and extraversion with intersubject coherence was examined. There were four male-male pairs, five female-female pairs, and two male-female pairs. The age of the participants ranged between 18 and 28 (M = 22.3, SD = 2.9). During the counterbalanced joint and individual conditions, the same mindfulness listening tape (3 min) was played, while during the individual task, a screen was placed between the two participants. Results showed an increase in intersubject coherence during joint practice compared to individual practice in frontal (F8) and temporal (T5 and T6) electrodes in the alpha band. With respect to personality characteristics, higher agreeableness of a dyad was associated with an increase in intersubject coherence in in temporal (T6) theta band. The increase in intersubject coherence in the theta band in high agreeableness subjects during joint practice might be associated with theory of mind activation. This study provides new insights concerning brain coherence in healthy people during joint mindful breathing, including the association with personality characteristics.