Mindfulness-based cognitive therapy (MBCT) is an effective treatment for reducing depressive relapse as well as residual depressive symptoms among adults with recurrent depression but the specific mechanisms through which this treatment works have yet to be examined. This study investigated MBCT's immediate (pre to post) effects on depressive symptoms and its potential theory-driven change mechanisms in a wait-list randomized control trial. Recurrently depressed patients, the majority of them in partial remission, were randomized to either an 8-week MBCT group (N = 26) or a wait-list control group (N = 19). Participants completed measures of depressive symptoms as well as measures of rumination and trait mindfulness before and after the intervention. Consistent with the MBCT change theory, bootstrapping-based mediation analyses demonstrated that reductions in brooding (an aspect of rumination) and increases in mindfulness independently and uniquely (accounting for other mediators) mediated the effects of the intervention on depressive symptoms. Given the pre-post design and the lack of temporal precedence, these findings provide initial evidence supporting the notion that mindfulness and brooding may be important change processes through which MBCT affects depressive outcomes.