Treatment for breast cancer affects psychological adaptation and related neuroendocrine stress indicators. Previously, a 10-week cognitive behavioral stress management (CBSM) group intervention decreased cortisol over 12-months among women receiving treatment for breast cancer. The current re-analysis tested whether changes in stress management skills at 6-month follow-up predict the magnitude of cortisol reductions at 12-months in a time-lagged analysis. Women (N = 128) with non-metastatic breast cancer recruited post-surgery were randomized to the CBSM intervention or 1-day psychoeducational seminar. Participants reported perceived CBSM skills and provided late afternoon blood samples for serum cortisol at baseline and 6- and 12-month follow-ups. Improved perceived ability to relax and use cognitive reappraisal skills at 6-months statistically mediated intervention-associated cortisol reduction from 6- to 12-months. This is the first study showing that improved perceived CBSM skills predict the magnitude of cortisol reductions over 1 year in this population and may guide development of more focused interventions.