Childhood obesity is a global public health challenge. Early prevention, particularly during the first 1000 days, is advocated. Health professionals have a role to play in obesity prevention efforts, in part due to the multiple routine contacts they have with parents. We synthesized the evidence for the effectiveness of obesity prevention interventions delivered by health professionals during this time period, as reviews to date have not examined effectiveness by intervention provider. We also explored what behaviour change theories and/or techniques were associated with more effective intervention outcomes. Eleven electronic databases and three trial registers were searched from inception to 04 April 2019. A total of 180 studies, describing 39 trials involving 46 intervention arms, were included. While the number of interventions has grown considerably, we found some evidence for the effectiveness of health professional-delivered interventions during the first 1000 days. Only four interventions were effective on a primary (adiposity/weight) and secondary (behavioural) outcome measure. Twenty-two were effective on a behavioural outcome only. Several methodological limitations were noted, impacting on efforts to establish the active ingredients of interventions. Future work should focus on the conduct and reporting of interventions.