Few weight loss and weight loss maintenance interventions are tailored to include factors demonstrated to predict the user's behavior. Establishing the feasibility and acceptability of such interventions is crucial. The aim of this study was to assess the acceptability and feasibility of a theory-based, tailored, online-delivered weight loss and weight loss maintenance intervention (Choosing Health). We conducted a mixed methods process evaluation of the Choosing Health tailored intervention, nested in a randomized controlled trial (N = 288) with an embedded N-of-1 study, investigating participants' and implementers' experiences related to intervention context, implementation, and mechanisms of impact. Measures included: (i) surveys, (ii) data-prompted interviews (DPIs) with study participants, (iii) semi-structured interviews with implementers, and (iv) intervention access and engagement data. Five themes described the acceptability of the intervention to participants: (i) monitoring behavior change and personal progress to better understand the weight management process, (ii) working collaboratively with the intervention implementers to achieve participants' goals, (iii) perceived benefits of non-judgmental and problem-solving tone of the intervention, (iv) changes in personal perception of the weight management process due to intervention tailoring, and (v) insufficient intervention content tailoring. The intervention delivery was feasible, however, emails and text messages differed in terms of accessibility and resources required to deliver the content. The use of Ecological Momentary Assessment as a technique to gather personal data for further tailoring was acceptable, and facilitated behavior change monitoring. Personalization of the intervention content above and beyond domain-specific issues, for example, by addressing participants' social roles may better match their needs. Support from the implementers and feedback on body composition changes may increase participants' engagement.