To describe the development of an online mindfulness-based intervention (MBI) to reduce anxiety before a first-time screening colonoscopy among average-risk patients. A qualitative study used an iterative process guided by health behavior and mindfulness theories and feedback from a convenience sample of patients, endoscopy medical staff, and community members. Patient and medical staff (n = 18) were included in formative interviews (30-45 min), eight helped during intervention development sessions (15-90 min), and four community members reviewed the MBI in individual sessions (60 min). Interviews and sessions were recorded, transcribed verbatim, and analyzed using NVivo qualitative data software. Two themes emerged from the study: (1) both patients and medical staff reported that average-risk patients have pre-procedural anxiety before a first-time screening colonoscopy, and (2) using stakeholder-engaged strategies in an iterative process with both patients and medical staff is important so the developed intervention is acceptable to the priority population and to ensure medical accuracy and avoid disruption of workflow. Using an iterative process with key stakeholders is essential to develop interventions that are feasible and acceptable. The MBI developed through this process is being compared to usual care in a pilot randomized controlled trial to determine intervention feasibility and patient acceptability and to collect preliminary efficacy data. If efficacious, the developed MBI has the potential to reduce pre-procedural anxiety which may improve patient behaviors (e.g., bowel prep adherence and quality), patient satisfaction, and clinic workflow by reducing cancellation/no-shows, the amount of sedation required, and procedural time.