U.S. industrialist and financier known, in conjunction with his older brother, John D. Rockefeller, for his role in the establishment and growth of the Standard Oil Company.

Rockefeller began his career as a bookkeeper. At age 21 he started his own business, Hughes and Rockefeller, as a produce commission merchant. In the mid-1860s John D. Rockefeller, who had invested in Ohio's newly discovered oil finds, asked William to head the export operations in New York City for what would in 1870 become Standard Oil.

Though he was not subject to as much public scrutiny as his brother, William played an active role in the company throughout its life as a “trust” and, later, as a holding company. He served as head of the New Jersey and New York branches until 1911, when the U.S. Supreme Court—acting on the Sherman Anti-Trust Law—dissolved the holding company. He retired in 1911, devoting his time to his investments and his railroad properties.