From Newsgroup: rec.arts.mystery
She grew up in Winnipeg and Montreal.
I'm surprised I can't find a longer obit.
https://www.legacy.com/obituaries/theglobeandmail/obituary.aspx?n=marianne-g-macdonald&pid=194973940
MARIANNE G. MacDONALD B.A., B. Lit, PHD 1934 - 2019 Passed away in London, England December 18, 2019 after a long illness. Born in Kenora, Ont., raised in Montreal, she attended McGill University before gaining a scholarship to Oxford University. Dr. Macdonald was a noted specialist on the life and works of the Poet Ezra Pound. She was the author of several books for children and the Dido Hoare mystery series. Predeceased by Erik Korn; she is survived by their sons, David (Wendy) and Andrew (Lea) Korn of London, England; as well as her sister, Sandra Macdonald of Toronto.
http://www.fantasticfiction.co.uk/m/marianne-macdonald/
(photo and book covers - it says her first children's book was published when she was 16!)
https://groups.google.com/g/rec.arts.books.childrens/c/J2qwz32951g/m/KoTkoNx3AXcJ
(birthday post, with booklist and many links)
Not to be confused with the slightly younger Chicago-born classics scholar/historian/playwright/philanthropist Marianne McDonald, OR the 73-year-old Michigan-raised juvenile writer Maryann MacDonald (note the spelling for both writers).
https://www.encyclopedia.com/arts/educational-magazines/macdonald-marianne-1934
(resume, interview, and article on her work)
https://www.mysteryscenemag.com/article/2244-dido-hoare-antiquarian-sleuth (about the Dido Hoare series)
https://www.kirkusreviews.com/author/marianne-macdonald/
(seven Kirkus reviews)
https://www.publishersweekly.com/pw/authorpage/marianne-macdonald.html
(six PW reviews)
https://www.goodreads.com/author/show/289243.Marianne_Macdonald
(reader reviews)
Lenona.
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