Great question, momtofive. Maybe someone who has done more research can chime in, but here is what I’ve found.
Homeschooling has been going on for centuries, in the strict sense of the phrase. So in that sense, no, Charlotte Mason did not found homeschooling. She did, however, start a movement that focused attention on and gave help to homeschool parents. In her day, many of the upper-class children were educated at home. Charlotte’s insights, methods, and philosophy added a new dimension to their education and brought life back to many home schoolrooms (as well as school classrooms).
She offered practical help to parents who were educating their children at home in her “Home Education” writings, and eventually so many parents were encouraged and got involved that they formed the PNEU (Parents National Education Union) with its monthly magazine, The Parents Review. By the 1920s Charlotte’s work had expanded to encompass a teacher college, several schools, and a correspondence school that sent out notes and advice to parents and governesses who were teaching at home.