Background Alcohol consumption is a major modifiable risk factor for female breast cancer, even in small amounts. However, awareness of this risk remains low. National breast screening programs are uniquely positioned to provide timely and targeted health information and behavior change strategies to improve alcohol literacy and reduce consumption. A breast screening service is a novel health care setting for brief alcohol intervention, with the potential for extensive reach. Objective This study aimed to conduct a formative evaluation with breast screening service consumers to understand the need for, and acceptability of, brief alcohol intervention in the breast screening setting and collaboratively design a brief alcohol intervention (Health4Her); to test the effectiveness of Health4Her in improving knowledge of alcohol as a breast cancer risk factor (primary outcome), improving alcohol literacy, and reducing consumption among women attending a breast screening service; and to examine the implementation strategy through process evaluation. Methods This was a hybrid type II effectiveness-implementation trial comprising a randomized controlled trial (RCT) alongside a mixed methods program evaluation guided by applicable elements of the Reach, Effectiveness, Adoption, Implementation, and Maintenance framework and Consolidated Framework for Implementation Research. Formative evaluation comprised a retrospective analysis of alcohol consumption data (n=49,240), a web-based survey (n=391), and focus groups and interviews (n=31) with breast screening service consumers. Women attending routine mammography, drinking at any level, were recruited to the single-site, double-blind RCT (n=558), and completed a baseline assessment before randomization (1:1) to receive Health4Her (alcohol brief intervention + lifestyle information) or control (lifestyle information) via animation on an iPad. Follow-up assessments were performed 4 and 12 weeks after randomization. The process evaluation included evaluation of trial administrative data, participant quantitative (n=497) and qualitative feedback (n=30), and site staff qualitative feedback (n=11). Results This research was funded in March and May 2019. Data collection for the formative evaluation and trial recruitment occurred between January and April 2020 and February and August 2021, respectively, with finalization of follow-up data collection in December 2021. Quantitative process evaluation data were collected during trial implementation, and collection of participant and staff feedback was finalized in December 2021. Results of the retrospective analysis of alcohol consumption data from breast screening service consumers is anticipated to be published in March 2023 and the results of the RCT to be published in March 2023. Conclusions This study is anticipated to generate new substantial knowledge on the alcohol consumption and literacy needs of women attending breast screening and the extent to which these can be addressed using a novel, tailored brief alcohol intervention. The study design permits the evaluation of the effectiveness and implementation of Health4Her to predict and facilitate uptake in breast screening services. Trial Registration ClinicalTrials.gov NCT04715516; https://clinicaltrials.gov/ct2/show/NCT04715516 International Registered Report Identifier (IRRID) RR1-10.2196/44867