Background: Mobile technology offers unique opportunities to reach people with health promotion interventions. Breastfeeding is an important public health issue, and fathers are a key support. Milk Man is a father-focused breastfeeding app that sought to engage fathers with information and conversation about breastfeeding, with the goal to impact positively on breastfeeding duration. Objective: The study aimed to describe the process evaluation of the Milk Man app that was trialed in the Parent Infant Feeding Initiative randomized controlled trial. Methods: The app used an information library, gamification, push notifications, and social connectivity via a Web-based conversation forum, which included polls and conversation starters, to engage fathers with breastfeeding information. Fathers had access to the app from approximately 32 weeks of gestation to 6 months postpartum. Process evaluation data were collected from a self-completed questionnaire administered via a Web-based link sent to participants at 6 weeks postpartum, and app analytics data were collected directly from the app. Quantitative data from both sources and qualitative responses to open-ended questions were used to triangulate findings to investigate patterns of usage and the effectiveness of each app engagement strategy to motivate and engage users. Results: A total of 80.3% (586/730) of participants, who were randomized to receive the app, downloaded Milk Man. Push notifications and interest in what other fathers had posted in the forum were the 2 main motivators to app use. Fathers used the app most while their partners were still pregnant and in the weeks immediately after the birth of their baby. Perspectives on the gamification strategy were varied. However, at 6 weeks postpartum, approximately one-third of fathers still using the app said that the gamification elements were encouraging the app use. The ease of use of the app and the design were important elements that were rated positively. The conversation forum emerged as the hub of app activity; all but 1 of the most accessed library articles and external organization links had been prompted as part of a conversation starter. Fathers posted comments in the conversation forum 1126 times (average of 2.21 per user) and voted in polls 3096 times (average of 6 per user). Conclusions: These results demonstrate that the Milk Man app was an acceptable source of breastfeeding information and support that fathers and fathers-to-be are prepared to use throughout the perinatal period. The app showed encouraging results with facilitating conversation between partners. The conversation forum was clearly central to the success of the app, and fathers provided suggestions for improvement. Gamification results were varied, yet it was a key motivator for some users. These results provide valuable insight into the acceptability of the engagement strategies, including motivations for use and user perspectives on the app. Trial Registration: Australian New Zealand Clinical Trials Registry ACTRN12614000605695; https://www.anzctr.org.au /Trial/Registration/TrialReview.aspx?ACTRN=12614000605695.