Jean became a member of the Communist Party of the United States (CPUSA) in the mid-1930s. This reflected her belief in social justice and her concerns about economic inequality and fascism, prevalent themes during the Great Depression and the rise of totalitarian regimes in Europe. Her interest in socialist and communist ideals came naturally in her circle of intellectuals and academics.

Jean was involved in various left-leaning organizations advocating for civil rights, labor rights, and anti-fascist causes. These activities aligned with her commitment to social change and her belief in the power of collective action. She would have been involved in organizing meetings, writing articles or pamphlets, and protesting injustice.

By the early 1940s, Jean's initial enthusiasm for the CPUSA began to wane. The Stalinist purges and the Nazi-Soviet Pact caused many intellectuals to question their allegiance to the Communist Party. She reportedly expressed concerns about the party's dogmatism and its suppression of individual freedoms. There are suggestions that she disengaged somewhat from active political involvement around this time, though her overall views likely remained left-leaning.

Jean’s political affiliations, along with her relationship with Oppenheimer, were viewed with suspicion during his security clearance hearings. While it was not the sole reason for the revocation of his clearance, it definitely contributed to doubts about his suitability for top-secret work, especially given the anti-communist fervor of the Cold War era. Her political past continued to haunt Oppenheimer throughout his career.