Do we know anything about Hogwarts in the 1880s? Ahh but if Mary is a witch she could be much older than she looks. The only timeline I could find jumps from 1492 to 1945.

Given the timeline, I could see Mary Poppins having been instrumental in the magical goings-on that paralleled WWI the way Dumbledore was in WWII.

Yup, Hal.

And that's a pretty nifty fanfic plot bunny right there.

I think Mary could even have been Hufflepuff - she's all about sticking with it and getting things picked up.

“A Gryffindor will jump off a cliff," Penelope said. Her tone suggested that it was a well-worn proverb. "A Slytherin will push someone else off. A Hufflepuff will call in five hundred other Hufflepuffs, and they'll carve a stairway. And a Ravenclaw -- " She winked at Madeleine Aerie, who joined in with her: "A Ravenclaw will get hold of a flying carpet."

D says Hufflepuff but I think she's a Ravenclaw. Isn't her umbrella handle a raven?

In the book, a fairly major pagan god is intimidated by her, as are some pretty powerful Shamans/Wizards. (The latter is an extremely racist chapter I try not to dwell upon.) She is Somebody; she is supernatural, powerful and mysterious. Definitely not Hufflepuff, cause Hufflepuffs are all about team efforts. Mary is a leader, never a follower, and most of the time solves stuff singlehandedly without even bothering to lead a team. Not Slytherin, because while she uses cleverness she is strongly into direct confrontation, and gives fair warning. That leaves Gryffindor and Ravenclaw. She is clever, a long term planner, makes heavy use of gadgets. So raven umbrella aside (which I don't remember for sure but am taking your word for) her character and modus operandi scream "Ravenclaw".

I am watching "Gone Baby Gone" and the movie is good but the casting is all wrong for the leads if you have read the books, especially Gennaro, she looks so wrong for the part.

Isn't her umbrella handle a raven?

Parrot in the movie.

Further thought on Mary Poppins. If you have to cram her into Hogwarts, then Ravenclaw fits best. But really she is a goddess, maybe not even a minor goddess, a force of nature. If her world intersects with Potter's world, she is more likely to be one of the forces the wizards have to deal with rather than something as humble as a mere wizard.

On the other hand, when properly summoned she chose to fill a role as humble as that of a Nanny (in the class society of the time a very humble role indeed) though she was a very unhumble Nanny. So she might choose to fulfill the role of a mere Hogwarts Professor under the right circumstances. But it would just be a cover role to let her carry out whatever her real mission was.