Cross-national studies of mental health are essential for understanding population differences and changes over time, but comparisons are often limited by the use of different assessment instruments across cohorts. Harmonising measures requires ensuring that constructs are conceptually equivalent and psychometrically comparable across cultural contexts. We developed and evaluated a theory-driven framework to harmonise depression and anxiety measures between ELSA-UK and ELSA-Brasil and assessed measurement invariance to determine whether meaningful comparisons could be made across cohorts and over time.