She was a leading muckraker, a term used to describe investigative journalists who exposed corruption and wrongdoing in American society in the early 20th century. Tarbell's most famous work was a series of articles published in McClure's Magazine from 1902 to 1904, titled "The History of the Standard Oil Company."
In this series, she meticulously documented the ways in which Rockefeller, through his Standard Oil Company, had used ruthless tactics to eliminate competition, including price wars, secret rebates, and even bribery.
Tarbell's work had a significant impact, raising public awareness of the abuses of big business and ultimately leading to the breakup of Standard Oil in 1911 by the Supreme Court.