The Green Sturgeon Acipenser medirostris is an anadromous, long-lived species that is distributed along the Pacific coast of North America. Green Sturgeon is vulnerable to global change due to its sensitive life history and few spawning locations. The persistence of Green Sturgeon is threatened by habitat modification, altered flows, and rising river temperatures. The southern Distinct Population Segment was listed as threatened under the U.S. Endangered Species Act in 2001 due to persistent stressors. Despite increased research efforts after the species was listed, substantial gaps in basic population information for Green Sturgeon remain. We present the only known information on age structure and growth of a threatened population of Green Sturgeon. By analyzing archived fin rays that were collected from 1984–2016, we revealed highly variable growth among individuals. We detected several age classes from 0–26 years and found similar growth rates of Sacramento River Green Sturgeon compared to northern populations. Though limited, this analysis is an important first step to understanding Green Sturgeon population dynamics and highlights critical research needs.