Alice Roth became a mathematics teacher after her Ph.D. in 1938, and only returned to research after her retirement in 1971. Her 1976 paper on the "fusion lemma" is said to have "influenced a new generation of mathematicians worldwide".
More reading: Alice in Switzerland: The life and mathematics of Alice Roth
Alice Roth remained at the Humboldtianum [high school] until her retirement in 1971. It appears that shortly before retirement she had begun her transition back to work in mathematics. After announcing her plans to return to research to friends and relatives, she was told by one of them that in his field of medicine it would be impossible to return after so long an absence. Surely, most mathematicians would agree that it is impossible in the field of mathematics as well.
And so Alice Roth would seem an unlikely candidate for success. Yet much had changed in the thirty years that she had been teaching. In particular, Roth's area of research – begun over thirty years earlier – had become fashionable. [...] At last Alice Roth had time on her side and was able to put her mathematical creativity to work. She was now "ant chnobble" (pondering a problem) full-time, gave talks to other mathematicians at universities, and made good progress – at the cutting edge of contemporary mathematics.
Roth's past as well as future work was to have a strong and lasting influence on mathematicians working in this area. Her Swiss cheese has been modified (to an entire variety of cheeses); the fusion lemma which appeared in her 1976 paper influenced a new generation of mathematicians worldwide.