A Culture Show programme follows Dr Grosvenor and presenter Alastair Sooke as they restore and re-examine the painting to identify its history.
Dr Grosvenor said: "To find a portrait by Van Dyck is rare enough, but to find one of his friendship portraits like this, of the wife of his best friend in England, is extraordinarily lucky.
"Although as part of our national heritage values are irrelevant, for insurance purposes it should now be valued at anything up to £1 million. Had it appeared at auction as a copy, and in its dirty state, it would probably only have been estimated at about £3,000-£5,000."
Adrian Jenkins, director of The Bowes Museum, said: "This is a thrilling attribution for The Bowes Museum and the end of a chain of events that began with the photography of the entire collection five years ago for the Public Catalogue Foundation. We are very grateful to Bendor for his work in identifying both the sitter and artist of this painting and re-instating it to its original glory."
Your Paintings: A Culture Show Special is on BBC Two at 6.30pm.