Purpose This study aims to evaluate the reach, usefulness, acceptability, and factors influencing engagement with a lifestylefocused text message intervention to support women's mental and physical health after breast cancer treatment. Methods This study uses a mixed-methods process evaluation nested in the EMPOWER-SMS randomised controlled trial (n = 160; intervention n = 80, wait-list control n = 80). Data sources included screening logs, text message delivery software analytics, intervention feedback survey, and focus groups (n = 16), which were summarised thematically based on the framework approach. Results A total of 387 women met the inclusion criteria (mean age ± SD = 59.3 ± 11.6 years). Participants who declined (n = 227) were significantly older than those who enrolled (n = 160; 62.2 ± 11.1 vs 55.1 ± 11.1 years, respectively, p < 0.001). Most intervention participants (64/80; 80%) completed the end-of-study survey, reporting the messages were easy to understand (64/64; 100%), useful (58/64; 91%), and motivating (43/64; 67%). The focus groups (n = 16) revealed five factors influencing engagement: (i) feelings of support/continued care, (ii) convenience/flexibility of message delivery, (iii) weblinks, (iv) information from a credible source, and (v) options to save or share messages. Conclusion A lifestyle-focused text message program was acceptable and useful for women after breast cancer treatment.However, text messaging may be a barrier for women aged over 68 years. Suggestions for program improvements included delivering the program to patients with other cancers, during all stages of treatment, and including more weblinks in text messages. Implications for Cancer Survivors Text message programs offer a low-cost way to deliver post-treatment health support to breast cancer survivors in a noninvasive way. Text messages can improve patient-health professional communication and were found to be acceptable and useful.