Oppenheimer was born on March 28, 1986, in Tulsa, Oklahoma, USA, to Reed and Gabrielle Oppenheimer. She has three siblings, two brothers, Eric and Luke, and one sister, Sophie. Her father studied history and anthropology at Yale University. Over the years, he has been affiliated with several charities, serving as the charter member of Hunger Project, founding chairman of the Grameen Foundation USA, and board member of Trees of Life and Chiapas International. Her mother has degrees in history (from Wheaton College) and art history (from Hunter College). She also has a doctorate degree from the Institute of Fine Arts, NYU. Gabrielle has served as an assistant director at the Brooklyn Children’s Museum. The family runs a private non-profit organization named The Reed Jules Oppenheimer Foundation (RJOF). Set up in 2012, RJOF promotes hunger and poverty reduction initiatives, education, environmental and human sustainability, and social justice, through wildlife conservation, agroecology, tree planting, microfinance, music festivals, personal empowerment strategies, and programs that elevate awareness. They are active in US, India, Bangladesh, Mexico, Guatemala, Nicaragua, Colombia, Paraguay, Honduras, Vietnam, Russia, Nigeria, Tanzania, Uganda, and Kenya.
In 2005, Oppenheimer graduated from Holland Hall and subsequently relocated to New York to study at the NYU’s Tisch School of the Arts, from where she received her degree in drama in 2009. She made regular appearances in local theatre productions and TV shows in Tulsa. When she was in New York, she considered pursuing a career in theatre. However, she eventually relocated to Los Angeles to work in films and television.
Sometime in 2015, Oppenheimer began dating Welsh actor Tom Ellis. In March 2017, the couple got engaged. Ellis was previously married to ‘EastEnders’ star Tamzin Outhwaite from 2006 to 2014. They have two daughters together, Marnie Mae and Florence Elsie. He has another daughter, named Nora, with former girlfriend Estelle Morgan.