Few studies have examined changes of diurnal cortisol profiles prospectively, in relation to non-pharmacological interventions such as mindfulness-based cognitive therapy (MBCT). Fifty-six patients remitted from recurrent depression (≥3 episodes) were included in an 8-week randomized controlled trial comparing MBCT plus treatment as usual (TAU) with TAU for depression relapse prophylaxis. Saliva samples (0, 15, 30, 45, 60 min post-awakening, 3 PM, 8 PM) were collected on six occasions (pre- and post-intervention, 3-, 6-, 9-, 12-month follow-up). Cortisol awakening response (CAR), average day exposure (AUCday) and diurnal slope were analyzed with mixed effects models (248 profiles, 1-6 per patient). MBCT (n = 28) and TAU groups (n = 28) did not significantly differ with respect to baseline variables. Intra-individual variability exceeded inter-individual variability for the CAR (62.2% vs. 32.5%), AUC(day) (30.9% vs. 23.6%) and diurnal slope (51.0% vs. 34.2%). No time, group and time by group effect was observed for the CAR and diurnal slope. A significant time effect (p = 0.003) was detected for AUCday, which was explained by seasonal variations (p = 0.012). Later wake-up was associated with lower CAR (-11.7% per 1-hour later awakening, p < 0.001) and lower AUCday (-4.5%, p = 0.014). Longer depression history was associated with dampened CAR (-15.2% per 10-year longer illness, p = 0.003) and lower AUCday (-8.8%, p = 0.011). Unchanged cortisol secretion patterns following participation in MBCT should be interpreted with regard to large unexplained variability, similar relapse rates in both groups and study limitations. Further research is needed to address the scar hypothesis of diminished HPA activity with a longer, chronic course of depression.