The subway system in the city of Seoul has dramatically evolved from a single subway line of less than 10 km in the early 1970s to one of the largest mass transit systems in the world, with more than 13 lines and 400 stations in 2014. This study aims to explore longitudinal changes in network accessibility and reliability in relation to the four evolutionary stages of the Seoul subway system (1979, 1985, 2001, and 2014). With rapid expansion of the network, accessibility and reliability have improved over time, but at a different pace and with different spatial patterns. The accessibility level has consistently increased, along with the core-to-periphery improvement spatial pattern, while reliability has been quickly enhanced as a result of the completion of a circular line in the second stage and stabilized early since the third stage. This study contributes to the field of transport network planning, in which well-balanced network functionality is a critical concern.