Understanding how social experiences throughout life shape later loneliness levels may help to identify how to alleviate loneliness at later lifestages. This study investigates the association between social relationship adversities throughout the lifecourse and loneliness in later life. Using prospective data from the Medical Research Council National Survey of Health and Development (N = 2,453), we conducted multivariable analyses to investigate independent, cumulative and moderated effects between the number of social relationship adversities experienced in childhood, mid-adulthood and later adulthood and the feeling of loneliness at age 68. We examined interactions between social relationship adversities and current quantity and quality aspects of social relationships. We found evidence of a step-dose response where greater exposure to social relationship adversities experienced at three earlier lifestages predicted higher loneliness levels in later life with more recent social relationship adversities more strongly related to loneliness. The results also demonstrated support for exacerbation and amelioration of earlier adverse social relationship experiences by current social isolation and relationship quality, respectively. This study suggests that social relationship adversities experienced throughout the lifecourse continue to influence loneliness levels much later in life. A key finding is that adverse social relationship experiences in earlier life may explain why otherwise socially similar individuals differ in their levels of loneliness. Implications for policy and research are discussed.