If I remember correctly, the first time I encountered a fictional female U.S. President - was in the novel "Alas Babylon," by Pat Frank, written, I think, in the very early 1960s. She succeeded to the presidency when the elected president was killed in a nuclear strike against Washington, I think. And I believe she was a fairly low-ranked officer in the federal government - maybe not even in the Cabinet, before becoming commander in chief.
Was there a fictional female US president before that?