During his tenure at MSU, his discoveries included the cause of deformities and lethality in birds of the Great Lakes, photo-enhanced toxicity, and the presence of perfluorinated chemicals in the environment, an important new class of contaminants widely used in common commercial products.
[10] In 2010, Giesy was elected to the Royal Society of Canada (RSC) for being "among the world’s most influential environmental toxicologists whose work in ecological risk assessment, including aquatic, wildlife and avian toxicology, has had global impact.
[11] In 2012, Giesy was recognized with the Lifetime Achievement Award from the Paris-based Scientific Committee on Problems of the Environment and China’s Zhongyu Environmental Technologies Corporation.
"[11] As a result of the COVID-19 pandemic, and like many US municipalities, Giesy helped develop an early warning system for the coronavirus by sampling and testing the city’s wastewater.
[17][18][19] Attorneys representing the state of Minnesota claimed Giesy was part of 3M's alleged campaign to "distort" and "suppress" scientific research on the toxicity of per- and polyfluoroalkyl substances.
[20] Giesy subsequently denied all of the allegations and argued that they were "an attempt by the State of Minnesota and its Attorney General, Lori Swanson, to smear his reputation after he declined to serve as an expert for them in a lawsuit against 3M.