ABSTRACT Social engagement has powerful effects on wellbeing, but variation in individual engagement throughout the lifecourse is wide. The trajectories may differ by gender and be affected by socio-economic status (SES). However, long-term development of social engagement is little studied and the effect of childhood SES on later-life social engagement remains obscure. We aimed to describe the social engagement development from childhood to middle age by gender and test the effect of childhood SES on middle age social engagement. Data (N=16,440, 51.3% male) are drawn from the on-going National Child Development Study, following British babies born in 1958. Social engagement was measured by social activities, voluntary work and social contacts, with follow-ups at age 11, 16, 23 and 50. SES was measured by father's occupational social class and tenure status. Structural equation modelling suggested inter-individual stability in social engagement, showing that development of social engagement started in childhood and increased social engagement in middle age through adolescence and early adulthood. Longitudinal effects were detected within and across the social engagement domains. Lower childhood SES was significantly related to a lower level of voluntary work and social activity in middle age, but to higher levels of social contacts. Although stability in social engagement is moderate over the lifecourse, variation within and across the different social engagement domains is shaped by differences in childhood SES.