Older adults are at a high risk for loneliness, and community-based interventions can help reduce loneliness for all older adults in a community, regardless of their migration status. However, little research has investigated how older adults, including locals and migrants (in this case, internal newcomers and international expats) participate in these interventions. The “Neighbourhoods in Solidarity” (NS) are a series of community-based interventions that aim to increase social connectedness and reduce loneliness in older adults (55+) in the canton of Vaud, Switzerland. This longitudinal embedded mixed-methods study aimed to understand whether older adults (distinguishing between locals, newcomers, and expats) were aware of and participated in the NS, to assess whether participation was associated with changes in loneliness, and to identify relevant processes that could explain a reduction in loneliness. We combined a longitudinal pre/post survey (235 respondents) with ethnographic observations and informal interviews. Quantitative findings showed that individuals who participated in the NS did not have significant changes in loneliness. Qualitative findings showed that perceived migration played an important role in who participated, and that the community distinguished between two types of migrants: newcomers who spoke French fluently, and expats who did not. Individuals were only ‘local’ if they had ancestors from the town. Some newcomers and some locals used the NS as a platform to build a new sense of community within the NS, whereas expats rarely participated. This was due to linguistic and cultural determinants, institutional constraints, interpersonal relationships, and personal preferences.