The paper aims to empirically analyze the effect of the quality of the formal institutional environment and social capital on financial development and long-run economic growth for a sample of developed and developing countries using the Generalized Method of Moments (GMM) for the period spanning from 1980 to 2009. The author sets out to find out if there is a synergetic effect between financial sector development and: first, formal institutional aspects measured by the institutional environment quality index, second, informal institutional aspects measured by the level of social capital in society. The article examines if the qualities of formal institutions as well as the level of confidence and cooperation between individuals are important to promoting financial sector and consequently long-run economic growth. The main results of the model are that (i) the development of formal institutions with a higher level of general institutional quality has a positive impact on long-run economic performance through the development of financial markets; (ii) the effect of financial sector development on long-run economic growth also depends on the state of informal institutions with a higher level of trust between individuals.