Rising obesity rates among young children and disproportionate effects on Māori, Pacific and socioeconomically deprived children require preventative action that is equitable and effective. As most children in NZ attend some form of early childhood care (96%) (kindergarten, Playcentre, Te Kōhanga Reo, Punanga Reo, Pacific language nests or education and care) for an average of 22 hours each week, there is huge potential to create a favourable food and physical activity environment for young children that supports good health outcomes and reaches those children most in need. Research showing effective translation of evidence-based approaches into public health practice for obesity prevention is scarce, especially for young children. The aim of this body of work was to determine if and how the Project Energize model, tailored for the early childhood setting (Under 5 Energize [U5E]), was effective at improving the nutrition and physical activity environments of early childhood centres (ECC) for young children in the Waikato region. The reach, effectiveness, adoption; implementation and maintenance (RE-AIM) of U5E were measured using a mixed method approach over a three-year period, August 2013–July 2016. U5E uses change agents, or Energizers, working with approximately 30 ECC each, delivering nutrition and fundamental skills workshops, resources, and policy development support, tailored to each centre. Data collection involved a pre- and post-audit of food and physical activity environment factors in 2013 and 2015 (implementation), a parent awareness survey in 2016 (effectiveness) and focus groups (U5E programme staff) and interviews (ECC managers or educators) at two time points (10 months apart) added to the understanding of information (Reach and Adoption, Effectiveness and Maintenance). ECC U5E action plan topics were counted and compared at six and 12 months post-programme initiation (implementation). Reach and adoption were assessed by a count of participating centres and comparisons with national and regional Ministry of Education statistics for numbers, equity, ethnicity and type of centre and geographical mapping of the rurality, socioeconomic status and ethnicity of the children at each centre. The reach and adoption of the identified target population was (93%, n = 133) and the equity, ethnicity and type of centres participating matched Ministry of Health contract targets of intervening for priority groups. Programme staff reported developing and moulding U5E to ensure programme alignment and building relationships with centres. Centres became involved in U5E because the programme ‘fitted’ centre philosophies, policies and practices, complemented their way of working, contributed health benefits for children and provided a positive reason to connect with their communities. The Energizers ‘way of working’ with centres, skills and knowledge appeared to greatly enhance programme engagement while the barriers to engagement in U5E were few. Between 2013 and 2015 the educator reported frequency of consumption (serves per day) of everyday foods increased significantly (p < 0.05) [vegetables (+0.203), fruit (+0.134), sandwiches (+0.116), and reduced fat milk (+0.106)] and occasional foods and beverages decreased significantly [flavoured noodles (-0.149), sweetened drinks (-0.080), potato chips (-0.206), cakes (-0.166), fruit juice (-0.064)), flavoured milk (-0.084), muesli bars (-0.130), muffins (-0.121), chocolate and sweets(-0.070)]. At follow-up, 25 of 87 centres had shifted from having either no or non-written expectations about physical activity to a written physical activity guideline or policy at follow-up and about 15 centres reported a similar shift in documentation of food and nutrition policy. In the first six months, the 121 enrolled centres set, on average, eight goals per centre in action plans, four nutrition and four physical activity goals reducing to five in the second six months. One-third of the nutrition and physical activity goals were achieved in the first six months of U5E, with a further 50% of the goals recorded by the Under 5 Energizers as initiated and being actioned in an ongoing way. Professional development for the educators was the most frequent goal in ECC action plans for both nutrition and physical activity. Just over half (45 of 78) the parents surveyed in 10 centres had heard of U5E and of these 33 recalled the main aims of U5E accurately. The length of time of U5E delivery and awareness of the programme were positively associated (χ² = 4.7151, p = 0.0299). The main healthy eating and physical activity messages parents had heard or seen in the centre were ‘less sugary drinks’ and ‘healthy lunchboxes’, with 40 parents reporting these messages, and 34 said that they had made changes to food at home or to food provided to children at the centre based on the U5E kaupapa. In contrast, changes to physical activity at home were reported by only a third of parents (n = 14). By the end of 2015, U5E appeared embedded in participating centres’ planning, budgeting and teaching programmes or activities/actions, with educators describing this as ‘part of what we do now’. Educators felt U5E assisted centres with achieving child development goals (through fundamental movement skills training) and aligned with the Te Whāriki curriculum strands. U5E seemed to also provide accountability in nutrition and physical activity for the centres. Reduced reliance on Energizer support indicated U5E appeared to be progressing over time towards programme maintenance. In conclusion, U5E achieved high reach and adoption and was well implemented and effective at changing food and physical activity policies and routines within ECCs in a sustainable way. Tailoring, by the Energizers, of U5E in diverse geographical, delivery style and ethnic environments of ECCs was a key reason for the programme’s high reach, effectiveness, adoption, implementation and maintenance. Parental awareness and use of U5E nutrition and physical activity messages could be improved although the sample size was limited. Raising parental awareness and use of U5E nutrition and physical activity messages should be a future focus to increase the reach to families of children enrolled in participating ECC. Small changes over time have the potential to influence policy and practice norms within early childhood centres to those that encourage healthy food and physical activity behaviours of enrolled children.