The adoption and effective delivery of evidence-based interventions within “real-world” community-based, primary health care service settings are of crucial importance. In this paper, we explore the successes and challenges of implementing a new complex, group-based, early parenting intervention called the Parent and Infant (PIN) programme. This study involved a systematic analysis of the processes and factors that influence the implementation of the PIN programme; the analysis was guided and informed by the Implementation Outcome Framework and the Consolidated Framework for Implementation Research. A documentary review, alongside a series of one-to-one interviews and small group discussions with a range of stakeholders (n = 44), and 7 focus groups (n = 24) were used as data sources. Factors that promoted programme adoption, acceptability, and implementation feasibility included programme characteristics and stakeholder attitudes, as well as organisational and systems factors (e.g. leadership and collaboration). Key challenges to implementation success included engagement and adoption barriers. This research provides a useful and important example of real-world, theory-driven implementation research which helped to identify interrelated processes, factors, and contexts which shape and influence the implementation of early intervention and prevention programmes, removed for blind review.