PURPOSE There is limited information regarding the use of the geriatric assessment (GA) for older adults with cancer in developing countries. We aimed to describe geriatric oncology practice among Mexican oncology professionals and to identify barriers and facilitators for the implementation of GA into the routine care of older adults with cancer in Mexico. METHODS We conducted an explanatory sequential mixed-methods study. We administered an online survey to cancer specialists in Mexico about the routine use of GA and barriers for its use. We then conducted online semistructured interviews with survey respondents selected by their use of GA, expanding on barriers and facilitators for performing GA. Descriptive statistical analyses were performed for quantitative data; qualitative data were analyzed inductively through thematic analysis. We developed joint displays to integrate quantitative/qualitative results. RESULTS We obtained 196 survey responses: 37 physicians (18.9%) reported routinely performing a GA. Medical oncologists ( P = .002) and physicians seeing ≤ 10 patients/day ( P = .010) were more likely to use GA. The most frequent barriers for GA use were lack of qualified personnel (49%), limited knowledge (43.9%), and insufficient time (37.2%). In the interviews (n = 22), the limited availability of geriatricians was commonly mentioned. Respondents highlighted the lack of geriatric oncology knowledge among cancer specialists and geriatricians. Saturation of oncology services and a lack of effective referral pathways for GA were also common issues. Facilitators included availability of geriatricians, system/administrative facilitators, presence of a multidisciplinary team, and availability of geriatric oncology education. CONCLUSION The routine use of geriatric oncology principles in Mexico is limited by the availability of qualified personnel and by insufficient knowledge. An educational intervention could improve the implementation of GA in cancer care.