Soybean yields in the US have steadily increased throughout the past century due to advances made in breeding and management practices. Despite these historical gains, producers will face a significant challenge to provide sufficient food, fiber, and fuel to supply a growing world population, which raises the question of whether the current rate of yield increase will meet this challenge. Although there is a directed effort to increase yield potential through targeted biotechnological strategies, there is a knowledge gap on how traditional breeding has physiologically altered plants to achieve current soybean yields. In order to identify physiological changes that have contributed to yield improvement, 24 soybean cultivars with release dates spanning 1923 to 2007 were grown in Champaign, IL in 2011 and 2012. Physiological traits including leaf-level photosynthetic rate, season-long efficiencies of light interception, solar energy conversion, and carbon partitioning to seed were measured. Despite occasional increases in photosynthetic rate, season-long conversion of light energy into carbohydrates showed no trend with year of release in either 2011 or 2012. The efficiency of light interception by the canopy increased linearly by 10% and 17% with year of release, while harvest index also increased linearly by 22% and 41% in 2011 and 2012, respectively. Seed yield increased in both years at an average rate of 16.7 kg/ha and 32.1 kg/ha per year in 2011 and 2012. This study suggests that traditional breeding has minimally affected seasonal photosynthetic carbon uptake of soybean while light interception and harvest index have improved considerably, and are perhaps approaching theoretical maxima. We provide evidence that conversion efficiency has been unaffected by traditional breeding in these historical lines and is below the theoretical maximum, indicating that increasing photosynthesis and conversion efficiency may be a strategy for future improvements in soybean yield potential.