Major depressive episodes (MDEs) of major depressive (MDD) or bipolar disorders (BD) are frequently complicated by features of borderline personality disorder (BPD). Mixed features are a hallmark of BD and affective lability of BPD, and both may markedly influence illness course. However, direct comparisons of outcome of depression in MDD, BD, and BPD are scarce. In a cohort study based on stratified sampling, we diagnosed psychiatric MDE patients with SCID-I/P and SCID-II interviews and examined mixed symptoms using the Mix-MDE scale and borderline symptoms using the Borderline Personality Disorder Severity Index. During a six-month prospective follow-up, the MDE patients with MDD (n = 39), BD (n = 33), or BPD (n = 23) completed biweekly online assessments. Using life chart methodology, we divided the follow-up period into qualitatively different mood state periods. We investigated durations of mood episodes, times to first full symptomatic remission, and their predictors. Remission rates were similar in MDD, MDE/BD, and MDE/BPD patients. MDE/BD patients experienced more numerous and shorter distinct mood state periods during follow-up than the others. MDE/BD was associated with shorter (HR = 2.44, 95 % CI = 1.27–4.67) and dimensionally assessed BPD severity with longer time to first remission (HR = 0.95, 95 % CI = 0.91–1.00). Moderate sample size and follow-up duration. Course of illness over six months differs between the three depressive groups. Bipolar depressive patients have the most alternating course and the shortest time to first period of remission. Dimensionally assessed severity of BPD may predict longer time to remission from depression. •Prospective outcome study of depression in unipolar, bipolar and borderline patients•Time to remission shortest among bipolar and longest among borderline patients•Bipolar patients had the most variable illness course, but hypomanic switch was rare.•Severity of borderline features was related to longer time to remission.•Assessment of borderline features among depressive patients may be clinically useful.