Pierre had spent much of the 1890s working with the management of the Johnson Company, a steel firm partly owned by DuPont. The company's president, Arthur Moxham, trained him in cost accounting and financial management, and in 1899, frustrated with the conservative nature of DuPont's management, Pierre resigned to take over the steel firm.
When DuPont was put up for sale following President Eugene du Pont's death in 1902, Pierre purchased it along with his cousins, Alfred I. du Pont and T. Coleman du Pont. The three immediately set about bringing a host of other smaller powder firms under the DuPont wing. Pierre served as treasurer, executive vice president, and acting president during the absence of the ailing Coleman du Pont until 1914, and finally as president from 1915 until 1919.
As chief of financial operations, Pierre du Pont oversaw the restructuring of the company along modern corporate lines. He created a centralized hierarchical management structure, developed sophisticated accounting and market forecasting techniques, and pushed for diversification and increasing emphasis on research and development. He also introduced the principle of return on investment, a key modern management technique. From 1902 to 1914, Pierre kept a firm rein on the company's growth, but with the onset of World War I he guided DuPont through a period of breakneck expansion financed by advance payments on Allied munitions contracts.
Pierre invested in General Motors (GM) and DuPont eventually owned 37 percent of the company. When GM faced bankruptcy in 1920, Pierre left DuPont to take over the automaker. Working with Alfred P. Sloan, Jr., Pierre created a decentralized management structure to cope effectively with the company's widely varying products and markets, an approach adopted at DuPont just a year later. By the time he retired from the Board of Directors in 1928, GM was the largest corporation in the world.
Pierre du Pont served as chairman of DuPont's Board of Directors until 1940. In retirement he demonstrated continued concern for the public welfare, opening his carefully cultivated estate, to the public. Pierre also served on the Delaware State Board of Education, helping finance school construction and pushing for higher public education standards.