Purpose Reduced stress and reduced risk of cancer recurrence are among the many benefits of physical activity (PA) for cancer survivors. Exercise behaviors are linked to motivational factors. We investigated the associations between motivational profile, self-reported levels of PA and stress and mental functioning in 94 post-treatment breast cancer survivors who voluntarily enrolled in an exercise program. Methods Participants completed Apter Motivational Style Profile (AMSP), Life Time of Physical Activity (LTPA) Questionnaire, International Physical Activity Questionnaire (IPAQ), Medical Outcomes Short Form SF-36® (SF-36), Perceived Stress Scale (PSS) and provided 10 saliva specimens (to measure cortisol levels). PA levels were calculated in metabolic equivalent hours per week (MET-hrs/wk). Results Participants reported high levels of current and historical PA (M = 39.2 MET-hrs/wk, SD = 39.7; M = 14.2 MET-hrs/wk, SD = 15.4, respectively). They also reported high levels of stress (M = 33.6, SD = 4.5) coupled with low mental functionality as measured by SF-36 Mental Components Scale (MCS) (M = 44.4, SD = 8.8). PSS was negatively associated with MCS (r = −0.27, p = 0.009). Salivary cortisol was not associated with any measure. Participants had a conformist (“follow rules”) and alloic (“about others”) motivational profile. No motivational, exercise history or stress variables were associated with current PA. Conclusions As expected, participants reported higher levels of stress and lower mental functioning. Participants presented a unique motivational profile relative to the general population. Further research into the associations of motivation, exercise behaviors and stress are warranted.