Plunkett was born in New Carlisle, Ohio and attended Manchester College (BA 1932 chemistry) and Ohio State University (Ph.D. chemistry 1936). In 1936 he was hired as a research chemist by E.I. du Pont de Nemours & Company at their Jackson Laboratory in Deepwater, New Jersey. On April 6, 1938 Plunkett checked a frozen, compressed 100 pound container of tetrafluoroethylene, used in chlorofluorocarbon refrigerant production. When he opened the valve to the container to remove an amount for chlorination (using HCl), Plunkett discovered that nothing came out. When he checked to see why, he discovered that a white powder had formed which did not adhere to the container. The tetrafluoroehtylene in the container had polymerized into polytetrafluoroethylene (Teflon), a waxy solid with amazing properties such as resistance to corrosion, low surface friction, and high heat resistance.
He was the chief chemist involved in the production of the gasoline additive Tetra-ethyl lead at DuPont's Chambers Works from 1939 to 1952. After that he directed Freon® production at DuPont before retiring in 1975. He was inducted to the Plastics Hall of Fame in 1973 and the Inventors Hall of Fame in 1985. Plunkett died on May 12, 1994 at the age of 84.