BackgroundThe Warwick Edinburgh Mental Wellbeing Scale (WEMWBS) is validated for measuring mental wellbeing in populations aged 11 + and has been translated into 30 + languages. The aims of this study were a) to translate and validate WEMWBS for use in Swahili-speaking populations to facilitate measurement and understanding of wellbeing, evaluation of policy and practice, and enable international comparisons; and b) to examine sociodemographic characteristics associated with higher and lower mental wellbeing in participants in the Girls’ Education Challenge (GEC) project in Tanzania.MethodsA short questionnaire including WEMWBS and similar scales for comparison, socio-demographic information, and self-reported health was translated into Swahili using gold standard methodology. This questionnaire was used to collect data from secondary school students, learner guides, teacher mentors and teachers taking part in the GEC project in Tanzania. Focus groups were used to assess acceptability and comprehensibility of WEMWBS and conceptual understanding of mental wellbeing. These were audio-taped, transcribed and analysed thematically. Internal consistency of WEMWBS, correlation with comparator scales and confirmatory factor analysis were completed as quantitative validation. Finally, multivariable logistic regression was used to explore associations between individual characteristics and ‘high’ and ‘low’ mental wellbeing, defined as the highest and lowest quartile of WEMWBS scores.Results3052 students and 574 adults were recruited into the study. Participants reported that WEMWBS was understandable and relevant to their lives. Both WEMWBS and its short form met quantitative standards of reliability and validity, were correlated with comparator scales and met the criteria to determine a single factor structure. For students in the GEC supported government schools: mental wellbeing was higher in students in the final two ‘forms’ of school compared with the first two. In addition: being male, urban residence, the absence of markers of social marginality and better self-reported health were all significantly associated with better mental wellbeing. For adults, urban residence and better self-reported health were associated with better mental wellbeing.ConclusionsThe Swahili translation of WEMWBS is available for use. Further work to explore how to intervene to increase mental wellbeing in vulnerable GEC participants is needed.Supplementary InformationThe online version contains supplementary material available at 10.1186/s12955-023-02119-9.