<A HREF="https://en.wikipedia.org/wiki/Alice_Roth">Alice Roth</A> became a mathematics teacher after her Ph.D. in 1938, and only returned to research after her retirement in 1971. Her <A HREF="https://dx.doi.org/10.4153/CJM-1978-103-4">1976 paper</A> on the "fusion lemma" is said to have "influenced a new generation of mathematicians worldwide".

Further listening: <A HREF="https://youtu.be/JiStBE_rMLk">8 minute portrait</A>

Further reading: <A HREF="https://link.springer.com/article/10.1007/BF02984813">Alice in Switzerland: The life and mathematics of Alice Roth</A>

> 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.