Alive & Thrive (A&T) provides strategic technical assistance (TA) to develop effective policies; improve maternal, infant, and young child nutrition (MIYCN) programme design and implementationand enhance system capacity to sustain quality MIYCN service delivery at scale. A qualitative assessment was conducted using document review and stakeholder interviews (n = 79) to describe a selection of A&T's TA in six countries and systematically assess the contextual and TA process-related factors that influenced the results achieved and document the lessons learned about MIYCN TA design and implementation. To facilitate the selection of different types of TA, we classified TA into two levels of stakeholder engagement and intensity. Under the Technical Advisor TA category, we assessed A&T's support to strengthen national policy formulation, monitoring, and implementation of the International Code of Marketing of Breast-milk Substitutes. For Capacity Development TA, we assessed A&T support to scale-up maternal nutrition services and to increase strategic use of data. Factors important for TA provision included identifying and engaging with the right people, using evidence to support advocacy and decision-making, using multiple ways to strengthen capacity, developing packages of tools to support programme scale-up, and reinforcing feedback mechanisms to improve service provision and data quality. Challenges included shifts in the political context, poorly functioning health systems, and limited resources to replicate or sustain the progress made. Continued investment in evidence-based and practical TA that strengthens the institutionalization of nutrition across all stakeholders-including government, medical associations, civil societyand development partners-is essential. Future TA must support governments to strengthen system capacity for nutrition, including financial and human resource gaps that hamper full scale-up.