Peter O’Neill is a descendant of John D. Rockefeller, Sr., the founder of Standard Oil Company. O’Neill was born to Abby Milton O’Neill, the oldest grandchild of John D. Rockefeller, Jr., who ran many of the family’s investments and much of philanthropic work for decades. O’Neill is active in the Rockefeller family’s affairs, is a member of the Rockefeller Family Fund’s investment committee, and is a director of Rockefeller Capital Management.
O’Neill began his career as a therapist in East Harlem, New York, where he worked at the Boys and Girls Harbor for more than ten years in the 1990s and early 2000s. O’Neill earned a master of social work from Columbia University and also attended the Ackerman Institute for the Family where he earned a two-year post graduate degree. 
O’Neill is an active member of the Rockefeller family’s projects. From 2001 to 2004, he was president of the Rockefeller Family Fund and is a current member of the Rockefeller Family Fund’s investment committee as well as chair of the audit committee. He is also a director of Rockefeller Capital Management and a director of Meriwether Capital LLC, a private equity firm.  He is chair of Winrock International’s board of trustee’s governance committee and a member of its executive committee.  Winrock International, named for Winthop Rockefeller, is a nonprofit organization engaged in a variety of international agricultural and social development projects. 
In 2009, O’Neill joined other Rockefeller family members in encouraging ExxonMobil to split the company’s roles of CEO and chairman in addition to focusing on environmentalist-aligned sources of energy. O’Neill, who headed the Rockefeller Family committee dealing with ExxonMobil at the time, said he had the support of more than three-quarters of family members over the age of 21. 
Peter O’Neill is a descendant of John D. Rockefeller, Sr., the founder of the Standard Oil Company. His mother, Abby Milton O’Neill, was born to Abby Rockefeller Mauze and David Meriwether Milton, and she was one of John D. Rockefeller, Jr.’s grandchildren. Abby O’Neill was very involved in the family’s affairs. She took the reins of the Rockefeller family’s charitable work and investments by serving as chairman of Rockefeller Financial Services and Rockefeller and Company from 1998 to 2004, and trustee for almost twenty years, as well as serving as Chairman of the Rockefeller Brothers Fund. 
Peter is the youngest amongst his five siblings who include George D. O’Neill, who faced a public divorce from Amy Whittlesey, daughter of former ambassador to Switzerland Faith Whittlesey, in the early 2000s that threatened to expose secrets surrounding the Rockefeller family trusts.