Political parties have long been theorized as a key political institution that links society with the formal political system, and thus have often featured prominently in theories of policy-making and political economy. Curiously however, until recently party systems have received substantially less attention in theories seeking to explain inequality and redistribution, which predominantly focus on democratic institutions and the organization of the political left. This dissertation conceptualizes the relationship between party systems, social reform, inequality, and redistribution, advancing the argument that at least a portion of social policy trajectories, inequality, and income redistribution in any given country is attributable to dynamics within its party system - particularly their structure and institutionalization. Hegemonic party systems can initiate large waves of social reform and redistribution in their formative years but their redistributive prospects significantly wane the longer a hegemonic party remains in power, while two-party systems are unlikely to provide much, if any, alleviation of inequality. Multi-party systems provide the greatest potential to redistribute but the relationship is conditional on their relative institutionalization. Stable multi-party systems with institutionalized parties are more likely to exhibit both lower levels of inequality and redistribute more income, whereas inchoate counterparts are unlikely to be more redistributive than either hegemonic or two-party systems. I build and empirically support the theory through the utilization of two "pathway" cases of Brazil and Colombia, built upon foundations of within-case comparisons and primary archival and personal correspondence among elites and party members covering nearly a century in each country. I also rely on a bevy of quantitative analyses on income data covering historical and contemporary Latin America, as well as much of the contemporary democratic and authoritarian world, to test the external validity of the theory.