This dataset comprises whole blood mercury concentrations, sex, predicted age, snout-vent length, and body mass index in American alligators (Alligator mississippiensis) captured at Yawkey Wildlife Center, South Carolina, from 2010 to 2017. Funding for this study was provided by the U.S. Geological Survey (Cooperative Agreement nos. G12AC20329, G15AC00264) and the South Carolina Department of Natural Resources (Grant nos. 2009094, 20100899). Alligators are an effective sentinel species for Hg biomonitoring because they frequently occupy the top position within wetland food webs, are long-lived, and appear to exhibit long-term site fidelity. This suite of traits makes them amenable to long-term longitudinal sampling that is reflective of Hg in the surrounding environment. We investigated total mercury (THg) patterns in whole blood of adult alligators from a population in South Carolina, USA. Our objectives were to investigate demographic, individual, and temporal variation in THg bioaccumulation patterns, including previously-unexplored non-linear effects. Using recently developed growth models and auxiliary predicted age at first capture information, we were able to differentiated between age and size-related variation in mercury bioaccumulation, which was previously confounded due to long-held assumptions of indeterminate growth patterns.