Objectives To validate the cross-national psychometric properties of the Mental Health Quality of Life questionnaire (MHQoL) and to develop an open-source toolbox for its scoring, transformation and presentation. Design Secondary analysis of data from a multicentre international randomised controlled trial (EMPOWER). Setting Workplace settings in small-sized and mediumsized enterprises (SMEs) and public sector organisations in Finland, Spain and the UK. Participants The sample included 564 employees: 122 from Finland, 114 from Spain and 328 from the UK. Most were white-collar workers in SMEs or public organisations, mainly in public administration, manufacturing, health/life sciences or higher education. Women were the majority (56%-91% across countries), and mean age ranged from 43 to 48 years. Interventions No intervention was delivered for this analysis; data were drawn from baseline assessments. Primary and secondary outcome measures Primary outcomes were internal consistency and construct validity of the MHQoL, evaluated using Cronbach's alpha, measurement invariance testing and multilevel analyses of associations between MHQoL dimensions and its visual analogue scale (VAS). Secondary outcomes were convergent validity, assessed through correlations between MHQoL scores and other mental health and quality of life measures (EuroQol 5-Dimension 5-level questionnaire (EQ-5D-5L), Patient Health Questionnaire-9 (PHQ-9), Generalized Anxiety Disorder-7 (GAD-7), Insomnia Severity Index, Perceived Stress Scale-4 (PSS-4), Psychosocial Risk Scale, and World Health Organization Five Well-Being Index (WHO-5)). Results The MHQoL showed good internal consistency across countries, with Cronbach's alpha ranging from 0.741 in Finland to 0.806 in Spain (overall alpha=0.787). Measurement invariance across Finland, Spain and the UK supported construct validity. Multilevel regression analyses showed associations between MHQoL dimensions and the MHQoL-VAS, with strongest contributions from Self-Image, Daily Activities, Mood and Future. Convergent validity was supported by moderate to strong correlations between MHQoL, EQ-5D-5L and related mental health measures. An open-source R package and Shiny web application ('MHQoL Toolbox') were developed for scoring, transformation and visualisation Conclusions The MHQoL is a reliable and valid measure of mental health-related quality of life across countries. The MHQoL toolbox supports consistent, transparent implementation, facilitating use in research, clinical practice and economic evaluations. Trial registration number NCT04907604.