Mahoney received a bachelor's degree in Biology from Cornell University and earned her doctorate in Ecology and Evolutionary Biology from University of California, Santa Cruz. She has been a long-time, avid field ecologist and scientific diver and has worked in temperate, tropical, and polar systems. For her PhD research, she focused on spatial and temporal ecological patterns within the kelp forests of the Monterey Bay National Marine Sanctuary. 

After her PhD, Mahoney received the California Sea Grant State Fellowship and was placed at the State Coastal Conservancy as the Climate Ready Fellow. In this role she focused on sea level rise adaptation planning in the San Francisco Bay and outer coast areas and was an integral part in bringing the update to the Baylands Ecosystem Habitat Goals project to completion. From 2018-2020, Brenna worked as a California Sea Grant extension specialist as the San Francisco Bay Sentinel Site Coordinator.