Objective The present study sought to identify mediators underlying the effects of an education and a peer support intervention for women with breast cancer and to determine if the efficacy of a peer support intervention is moderated by cancer severity. Methods Participants included 180 patients with early stage (I or II) and 65 patients with late stage (IV) breast cancer. The study was originally planned as a 2 (early stage, late stage) × 3 (education intervention, peer support intervention, control condition) design; however, the education condition for the late stage cancer group was dropped, because of slow recruitment. Participants completed measures of well‐being prior to being randomized (Time 1), then again 2 weeks after the group meetings ended (Time 2), and 6 months later (Time 3). Results Among the participants who had attended at least one group meeting, the education intervention predicted more life purpose and marginally predicted more perceived physical health at Time 2. The peer support intervention predicted more life purpose and less depressive symptoms at Time 2. Cancer severity did not moderate these effects. The effect of the peer support intervention on depressive symptoms was mediated by life purpose. None of the intervention effects were evident at Time 3. Conclusions Peer support interventions have positive short‐term effects on well‐being, among women with late and early stage breast cancer, and these effects are partially mediated by changes in life purpose. Education interventions have positive short‐term effects on well‐being among women with early stage breast cancer. Copyright © 2015 John Wiley & Sons, Ltd.