Psychosocial stress in cancer survivors may contribute to compromised quality of life and negative cancer outcomes, which can be exacerbated by poor coping skills and emotional reactivity. Mindfulness based interventions (MBIs) have shown effectiveness in reducing stress, improving quality of life and coping skills in cancer survivors. We tested whether an MBI would also improve reactivity to an acute laboratory stress task. A total of 77 women with a cancer diagnosis were recruited for a waitlist-controlled trial of Mindfulness-Based Cancer Recovery (MBCR). Participants completed a laboratory-based psychosocial stress paradigm (the Trier Social Stress Test—TSST) pre- and post-intervention, throughout which cortisol and cardiovascular profiles were measured. Neither cortisol nor cardiovascular reactivity to the TSST was changed pre-to post intervention, either between or within groups. Blunted cortisol, but not cardiovascular, reactivity was observed across both groups, which may have contributed to the lack of intervention effect. Previous research suggests that diurnal cortisol is blunted following cancer treatment; the current findings suggest this blunting may also occur during exposure to acute stress.