OBJECTIVE: Explore experiences of women and care partners living with metastatic breast cancer (MBC) in the new environment of extended MBC survival. Care partner results are presented. DESIGN: Qualitative descriptive interviews with conventional content analysis. SAMPLE: Twelve care partners nominated by 1-to-5-year MBC survivors participating in the overall study. METHODS: Semi-structured interviews conducted over phone, Zoom, and in-person. FINDINGS: "Becoming a Co-survivor" entailed 5 categories: Meeting New Challenges, Changing Supportive Roles, Navigating Decisions, Emotional Toll, and Coping. Sixteen subcategories provided depth and dimension. Qualitative differences depicted journeying from heighted emotions to increasing expertise and mutual communication between survivors and care partners over time. CONCLUSIONS: MBC care partners possess varied characteristics, and experience challenges influenced by time since MBC diagnosis, necessitating further study. IMPLICATIONS FOR PSYCHOSOCIAL PROVIDERS: Acknowledge the variety of persons who assume care partner roles, assess needs, and design programs to address psychosocial challenges presenting along the survivorship continuum.