Colette Bourlier was awarded the mark of "high distinction" for her work, which she successfully defended on Tuesday before a jury of the University of Franche-Comte in Besancon, eastern France.
"It took a bit of time to write because I took breaks," Bourlier said, explaining the exceptional time for her opus.
The thesis was entitled "Immigrant workers in Besancon in the second half of the 20th century."
It drew on her own experience as a teacher in literacy programmes for immigrants in the eastern French city.
She became interested in studying for a PhD after she retired in 1983. Normally, a PhD in France is written in three years, but Bourlier was "an extremely atypical" student, said her professor, Serge Ormaux.
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"She is probably the only person who knew all the aspects in such detail and who was able to weave everything together. She backed it up with statistical analyses."