In the 16 months after the end of the Manhattan Project, Greenewalt served as Stine’s assistant, assistant director of the Development Department, and assistant general manager of the Pigments Department. In June 1946, he became a vice president and member of the Executive Committee, and in January 1948 he succeeded Walter S. Carpenter as president.
Greenewalt made the pursuit of large-scale research projects designed to produce “new nylons” DuPont’s major postwar objective. As part of this push, he convinced DuPont’s Executive Committee to spend $30 million on new research facilities and to fund university research programs. Between 1948 and 1962, Greenewalt oversaw the development of Orlon, Dacron, and Lycra synthetic fibers. In the latter part of his presidency, DuPont also began to expand overseas, establishing the International Department to coordinate global ventures.
By the late 1950s, however, DuPont had become overly reliant on textile fibers, faced slowing growth, and diminishing returns from textile research. In 1961 Greenewalt urged every department to focus on developing new markets and announced a program of diversification “beyond existing fields of interest and beyond chemistry.” Greenewalt stepped down from the presidency the next year, but continued to advocate the development of new product lines until his retirement from the Board of Directors in 1988