Research has demonstrated that life transitions lead to heightened experiences of loneliness, in part because they engender isolation. We tested whether the degree of isolation during a transition influences loneliness, and if this is due to their effects on social identity processes. Employing an experimental paradigm, Study 1 (N = 213) found that when the transition involved isolation from new networks, in this case studying at university online as opposed to in person, it led to more loneliness, and this was mediated through reduced social identification and continuity of group memberships. Study 2 (N = 215) replicated these effects using a different experimental paradigm in which the transition involved isolation from old group memberships, namely moving to a new neighbourhood from interstate. Study 3 (N = 2346) employed a quasi-experimental repeated-measures design to assess the impact of a highly isolating life transition on loneliness–COVID lockdown. Australians in prolonged lockdown experienced increases in loneliness and this was mediated through (lack of) continuity of group memberships. Overall, these results suggest that isolation needs to be considered when assessing the impact of life transitions on loneliness. Moreover, the mediation results indicate that isolating transitions may be responsible for loneliness because these make it difficult to maintain crucial group memberships and form a sense of identification with relevant new groups.