Despite growing concerns in the UK about social isolation, there remains a lack of data on the extent and time trends of social isolation from longitudinal, population-based studies. There is also little research that assesses the multiple domains of social isolation across the lifecourse and between generations in a holistic way accounting for different contexts. By applying a multi-context, multi-domain framework of social isolation to 5 successive British birth cohorts, we provide conceptual and empirical understanding of social isolation trajectories across the lifecourse and identify potential generational and sex differences in trends. Where data were available, comparable social isolation indicators were generated to enable lifecourse trajectories and cross-generational trends to be explored. Information on isolation was available across the following relational contexts: household i.e., living alone; partnership, family and friends outside the household; education and employment networks; and community engagement. Trajectories were modelled stratified by sex using a multilevel growth curve framework. Data were analysed from 73,847 individuals (48.5% female), in 5 successive cohorts born in 1946 (N = 5,362), 1958 (N = 16,742), 1970 (N = 16,950), 1989-90 (N = 15,562), and 2000-01 (N = 19,231). Exploring a range of social isolation indicators across several contexts provided a nuanced picture of social isolation across the lifecourse and between generations in the UK, with no consistent pattern of increased or decreased isolation over time. For example, more people are living alone, less women are out of education and employment in midlife, more people are volunteering, but fewer people regularly engage in religious activity. It therefore highlights the need to focus on a range of social isolation indicators across contexts to understand how people compensate for specific types of isolation, and to understand structural differences in social configurations in the UK, which may not only define the timing and sequencing of life transitions but also social isolation.