Mesoscale eddies are ubiquitous in the Southern Ocean particularly in regions where the Antarctic Circumpolar Current interacts with topography. Eddies play a critical role in the meridional transport of heat and salt across the Antarctic Circumpolar Current. However, observations of the heat and salt content of eddies are extremely rare. Here we present new observations that characterize the three‐dimensional structure of a cyclonic (cold‐core) eddy generated at the Subantarctic Front south of Tasmania. Automated detection software is used with satellite altimetry to follow the eddy through the Subantarctic Zone. The physical properties at the center of the eddy are substantially modified from those near the formation region, indicating a strong transformation during the eddy's lifetime. The eddy carried heat and salt content anomalies relative to surrounding Subantarctic Zone waters of −0.5 ± 0.1 × 10²⁰ J and −2.1 ± 0.4 × 10¹² kg, respectively. Previous studies have not captured the full heat and salt content of eddies due to limited observations and have underestimated their content by a factor of 2 to 3 south of Tasmania. Applying the observed correlation between eddy‐elevated volume and eddy heat and salt content to the history of cyclonic eddies in the satellite altimeter record leads us to propose that about 21% of the heat carried across the Subantarctic Front south of Tasmania is achieved by cyclonic eddies entering the Subantarctic Zone. The freshwater contribution to the Subantarctic Zone by long‐lived cold‐core eddies is of the same order of magnitude as the Ekman flux in this region.