Background: The present study investigates possible predictors of treatment response in an Internet-delivered Mindfulness-Based Cognitive Therapy (iMBCT) intervention with therapist support. This iMBCT program, a fully online delivered intervention with asynchronous therapist support, has previously been shown to be efficacious in reducing symptoms of anxiety and depression in women treated for breast cancer and men treated for prostate cancer. Methods: Eighty-two breast- and prostate cancer survivors experiencing psychological distress received 8 weeks of therapist-guided iMBCT. Primary outcomes were improvement in anxiety and depression scores from baseline to post-treatment and from baseline to six-months follow-up. Clinical predictors included levels of depression and anxiety at the time of screening and at baseline, as well as time since diagnosis. Demographic predictors included age and educational level. Therapy-related predictors included working alliance, self-compassion, and five facets of mindfulness. Mixed Linear Models were employed to test the prediction effects over time. Results: Higher levels of baseline depression were associated with increased treatment response in anxiety at post-treatment, and lower levels of self-compassion were associated with increased treatment response in depression at post-treatment. None of the proposed predictors significantly predicted treatment response at six-months follow-up. Conclusion: The findings suggest that iMBCT can be provided for cancer survivors regardless of their age, educational level, and time since diagnosis (up to five years) and that therapeutic alliance is not crucial for treatment response. We did not identify characteristics predicting treatment response, although many factors were tested. Still, other characteristics may be predictors, and given the relatively small sample size and a large number of statistical tests, the results should be interpreted with caution.