Despite being well-positioned to promote healthy lifestyles in young children, early years practitioners often face barriers to supporting child weight management. This mixed-methods study aimed to assess the preliminary effectiveness and acceptability of an e-learning toolkit (HealthyWEY) designed to upskill and support multi-agency professionals to promote healthy weight in early childhood. A total of 54 health visitors/community nursery nurses, 38 children's centre staff and 17 other health professionals engaged with the HealthyWEY e-learning, which drew on self-determination theory and consisted of nine modules that were completed over 7-10 weeks. Non-parametric statistical analysis using Wilcoxon's signed-rank tests were used to explore participants' practice-based knowledge, psychological needs satisfaction and motivations for prioritising pre-school child weight from pre- to post-intervention. Focus groups ( = 11) were conducted with a sub-sample of multi-agency professionals ( = 39) to explore the process of implementation across sites, while interviews were also conducted with two parents/carers who took part in consultations with HealthyWEY-trained practitioners. After completing the HealthyWEY e-learning, participants perceived fewer barriers to pre-school child weight management (median change = -0.7; < 0.001), greater autonomy (median change = 0.7, < 0.001), competence (median change = 0.8, < 0.001) and relatedness (median change = 0.4, < 0.001) and a higher autonomous motivation towards promoting healthy weight (median change = 0.3, < 0.001). E-learning was perceived to be highly relevant to participants' roles and congruent with local child weight strategies. Challenges to implementation included time constraints and disruptions related to the COVID-19 pandemic. Recommendations for a better user experience were offered by enhancing the toolkit's design and interactivity. Engagement with the HealthyWEY e-learning led to promising changes in perceived barriers and motivational variables. The toolkit was perceived to be acceptable amongst multi-agency workforces, albeit challenging to prioritise within time-pressured health and early years settings.