Not a pure mathematics example, but a famous example in the sciences is Nettie Stevens, a US geneticist who discovered sex chromosomes.

Born in 1861, she taught at a high school for many years before returning to further education and obtaining her Master's degree in Biology in 1900 and her PhD in 1903. She continued as Reader in experimental morphology for a year and worked at Bryn Mawr as an Associate in experimental morphology, publishing 38 articles (some of them of major significance in the field). She tragically died of breast cancer aged 50 in 1912.