Oligarchisation can be detrimental to democratic institutions. However, informal, inequality-based, patron-client relationships between the central and regional elites formed the backbone of many early-modern territorial states lacking legal and fiscal capacities to develop efficient, professional, and formal bureaucracies. We outline the crucial role inequality and clientelism played in the Polish-Lithuanian political system. We argue that the country’s parliamentary system depended on the ‘goldilocks’ level of inequality and power concentration to bind its republican, monarchic, and aristocratic components with a cliental structure. To support patronage, the state promoted the concentration of influence. We study a new, vast, genealogical dataset of family ties and public offices held by the Polish-Lithuanian elite. We employ the network analysis to measure long-term changes in the socio-political capital of the senatorial elite based on their position in the family-office network. We identify centralisation of influence in the hands of a few families as well as deepening inequality of influence amongst the senators. In time, too high levels of inequality subverted the coordinating role of the king, distorted the cliental system, and led to a factional conflict between the royal and oligarchic factions. This led to a parliamentary gridlock and petrification of central institutions of governance that ultimately led to state failure. High inequality can partially explain the collapse of the Polish-Lithuanian Commonwealth.