He was a prominent member of the Rockefeller family, contributing to its reputation as a leading American business dynasty.

In addition to elder brother John, William Jr.'s siblings were Lucy, Mary, and twins Franklin (Frank) and Frances (who died young).

He also had two elder half-sisters, Clorinda (who died young) and Cornelia, through his father's affairs with mistress and housekeeper Nancy Brown.

About two years later, he joined his older brother's produce commission business, Clark and Rockefeller, which later supplied provisions to the Union Army.

When John D. formed Rockefeller, Andrews & Flagler in 1867, he invited William to take charge of the company's export business in New York.

They had Amalgamated Copper Company buy Anaconda from them for $75 million in capital stock, which was conveniently printed for the purpose.

They paid back Daly for Anaconda and sold $75 million worth of stock in Amalgamated Copper to the public.

After Daly died in November 1900, banker John Dennis Ryan rose to head Amalgamated's Montana operations.

[3] When the newly formed Mutual Alliance Trust Company opened for business in New York on the Tuesday after June 29, 1902, there were 13 directors, including Emanuel Lehman and Rockefeller.

Her sister Esther Judson Goodsell was married to Oliver Burr Jennings, who became one of the original stockholders of Standard Oil.

[citation needed] In 1886, Rockefeller bought property in Westchester County along the Hudson River from General Lloyd Aspinwall.