This paper addresses an under-explored area of sociologically oriented health research, namely, the mental health and physical activity (PA) experiences of new fathers. Drawing upon responses to an online qualitative survey from 32 fathers, aged 18 or over, and living in the UK, we show how the decline in these fathers' overall PA was associated with poor mental health and the changing constraints that characterised their increasingly complex networks of interdependence. These constraints corresponded with shifts in fathers' PA engagement from team sports towards individualised, flexible, and more recreationally oriented lifestyle activities like running and the gym. Fathers' engagement in these activities appeared to exacerbate the complex feelings of guilt and isolation that they already encountered as new fathers. These experiences were simultaneously associated with feelings of shame associated with being insufficiently active and fearing judgement about their engagement in fathering responsibilities. The paper has important policy implications, highlighting the need for tailored support for new fathers in the perinatal period, and implications for practice, suggesting that co-produced community-based PA programmes are potentially effective settings for engaging new fathers in PA and promoting their mental health.