I haven’t as yet been able to find any biographical information on Constance Marshall, but the search continues. Constance Marshall wasn’t a prolific illustrator of pony books. She mostly appeared to work on Blackie’s Carol Vaughan titles, though didn’t do her last book, King of the Castle, which was published in 1968. She also illustrated at least the first two of Rosemary Manning’s popular Dragon books.
Finding the books:most are relatively easy to find.