Tatlock struggled with her sexuality, at one point writing to a friend that "there was a period when I thought I was homosexual. I still am, in a way, forced to believe it, but really, logically, I am sure that I can't be because of my un-masculinity." She began seeing Robert Oppenheimer in 1936, when she was a graduate student there and Oppenheimer was a professor of physics at Berkeley. They met through his landlady, Mary Ellen Washburn, who was also a member of the Communist party, when Washburn held a fund raiser for communist-backed Spanish Republicans. The couple started dating and had a passionate relationship; he proposed to her twice, but she refused. She is credited with introducing Oppenheimer to radical politics during the late 1930s, and to people involved with, or sympathetic to the Communist Party or related groups, such as Rudy Lambert and Thomas Addis. They continued seeing each other after he became involved with Kitty Harrison, whom he married on November 1, 1940. Oppenheimer and Tatlock spent the New Year together in 1941, and once met at Mark Hopkins hotel in San Francisco.