Chronic conditions are associated with large personal, familial and social costs, and have deleterious effects on individuals’ mental health. Drawing on the stress process model, we theorise and test how the presence of a partner moderates the extent to which living with a chronic condition affects mental health, and whether any protective effects change with time since illness onset, or differ between men and women. Our empirical analyses rely on nationally representative, panel data for Australia (n∼180,000 observations) and panel regression models. Being in a partnership, particularly in a marriage, is associated with better mental health amongst all individuals, but more so amongst the chronically ill. This advantage remains beyond the year of illness onset, and is of a comparable magnitude for men and women. These findings bear important implications for mental health in modern societies experiencing rapid population ageing, a rising prevalence of chronic illness, and declining marriage rates.