The middle Trinity River in Texas supports one of the premier trophy alligator gar (Atractosteus spatula) fisheries in the world. Published data on alligator gar life history and population characteristics are sparse, yet these data are needed to inform conservation and management. Using data from over 850 fish collected between 2007 and 2014, we described the size structure, population abundance, angler exploitation, and vital rates of this unique population. Collection of fish relied heavily on angler cooperators and included a three-year mark-recapture effort and the removal of sagittal otoliths from fish harvested by bow anglers. Size structure and population abundance data revealed why this population supports such a popular fishery. Size structure was broad (fish ranged from 46 to 241 cm) and trophy-sized alligator gar (> 180 cm) comprised more than 23% of the sample. Density of alligator gar was almost twice that reported for other systems and annual estimates of abundance of fish ≥107 cm ranged from 7903 to 8413 fish (26 fish river-km-1; 0.8 fish ha-1). Annual total mortality was estimated to be 8.5% of which angler exploitation did not exceed 5%. Individual length at age was highly variable; however, growth curves were generally similar to previous studies. While more restrictive regulations (e.g., harvest quotas or permits) may be required in the future if angling pressure increases, the middle Trinity River currently provides a sustainable trophy alligator gar fishery.