How was trust created and reinforced between the inhabitants of medieval and early modern cities? And how did the social foundations of trusting relationships change over time? Current research highlights the role of kinship, neighbourhood, and associations, particularly guilds, in creating ‘relationships of trust’ and social capital in the face of high levels of migration, mortality, and economic volatility, but tells us little about their relative importance or how they developed. We uncover a profound shift in the contribution of family and guilds to trust networks among the middling and elite of one of Europe's major cities, London, over three centuries, from the 1330s to the 1680s. We examine almost 15,000 networks of sureties created to secure orphans’ inheritances to measure the presence of trusting relationships connected by guild membership, family, and place. We uncover a profound increase in the role of kinship – a re-embedding of trust within the family – and a decline of the importance of shared guild membership in connecting Londoners who secured orphans’ inheritances together. These developments indicate a profound transformation in the social fabric of urban society.