The aim of the current study was to determine whether the Global Assessment of Functioning (GAF) score is associated with the functional exercise capacity among in- and outpatients with bipolar disorder. Sixty-five (36♀) persons with bipolar disorder performed a 6-minute walk test (6MWT) and were assessed with the GAF, Quick Inventory of Depressive Symptomatology Self Report (QIDS), and the International Physical Activity Questionnaire (IPAQ). The mean GAF-score was 55.0 ± 15.0, whereas the mean distance achieved on the 6MWT was 615.6 ± 118.6 m. There was a positive association between the GAF score and 6MWT score (r = 0.60, p < 0.001). A backward regression analysis demonstrated that an inpatient status, illness duration (16.1 ± 10.7 years), and the QIDS score (7.7 ± 5.7) explained 72.4% of the GAF-score variance. The GAF, QIDS score, and age explained 74.1% of the 6MWT-score variance. Our results indicate that a bidirectional relationship is evident between the exercise capacity and global functioning among people with bipolar disorder.