Slate notes that John D. Rockefeller was the world's first billionaire. He achieved this status in the early 1900s as a result of his investment in the Standard Oil Company. PBS notes that Rockefeller achieved the pinnacle of his financial success in 1912 when he earned a reported $900 million.
Despite the large amount of wealth Rockefeller accumulated over his lifetime, PBS notes that at the time of his death, Rockefeller's estate was worth only $26,410,837. He had given away the bulk of his estate to his heirs and philanthropic organizations, such as the Rockefeller Foundation, which he established in 1913 and to which he had given a reported $23 million by 1929.