Evaluating the impact of advocacy for policy change presents many challenges. Recent advances in the field of evaluation, such as contribution analysis (CA), offer guidance on how to make credible claims regarding such impact. The purposes of this article are (a) to detail the application of CA to assess the contribution of an advocacy initiative to improve infant and young child feeding policies and (b) to present the emergent theory of change and contribution story of how progress was achieved. An evaluation applying developmental evaluation and CA was conducted on the Alive & Thrive (A&T)–UNICEF initiative in seven Southeast Asian countries to document the extent to which policy objectives were achieved and identify key drivers of policy change. A contribution story was developed based on these experiences. The advocacy approach, which involved a four‐part process, contributed directly to (a) set the agenda of various actors and (b) create a strategic group; and indirectly to (a) set and maintain the issue on the agenda at all stages of the policy cycle, (b) support the government to carry out a set of critical tasks, and (c) extend commitment. All of this helped to achieve progress towards policy change. External influences were at play. The flexibility of A&T allowed key actors to utilize the positive external influences and address some of the negative ones through developing responsive strategies mitigating their effects. The emerging contribution story supports that A&T–UNICEF initiative contributed to the progress achieved in the participating countries.