- "Mrs. Macready?"
"I'm afraid so."
- ―Peter and Mrs. Macready [src]
Mrs. Macready was the housekeeper at Professor Kirke's House in the country. In addition to managing the staff there, (Ivy, Margaret and Betty), she also gave tours of the mansion (which was very old and famous), when sightseers arrived.
She was displeased that the Pevensies had come to stay in the mansion during the war, and had firmly instructed them to keep out of her way. One morning, the Pevensies heard her and a large group of people on one of her tours, and they attempted to hide from them, but no matter where they went, the visitors were always one step behind them.
Mrs. Macready eventually led the tour right up to the spare room, where the Pevensies hide in the Wardrobe, but stopped outside. When the Pevensies came out of the wardrobe (after spending a lifetime in Narnia), they heard Mrs. Macready and the visitors talking outside, where no time has passed.
Mrs. Macready is very proud of her position as Professor Kirke's housekeeper, and perhaps even more proud of the fine old house. She serves in the story primarily as a reason for the Pevensies to enter the wardrobe again, but also as a hostile influence, indicative of the children's unstable world. Her stern personality drives the children to hide in the wardrobe to escape her displeasure.
The interesting thing is that Mrs. Macready never learns of the role she plays in getting the Pevensies to Narnia. As far as she knew, she was simply giving another tour of the old house.
Mrs. Macready seemed to have a dislike for (or, at least, a stern demeanour around) children, and the Pevensies were always sure to keep out of her way. She was dedicated to the professor, in ensuring that his house and all the historical artifacts in it were kept in prime condition.
In the film, she drove a horse and buggy.
In the film adaption of the The Lion, the Witch and the Wardrobe, there was no mention of Mrs. Macready ever taking any tour groups around the mansion. She once controlled her temper when Professor Kirke saw Lucy crying and took her to the kitchen for a drink.
When the Pevensies all went to Narnia, they did so by accidentally breaking a window (they had been playing cricket; Edmund had hit the ball in the direction of the window by accident), and all ran through the house to get away from what they thought was Mrs. Macready. But everywhere they went, she always seemed to be one step ahead of them. Peter himself said, "She's faster than she looks!".
When they eventually returned from Narnia, Mrs. Macready was not in the room with them, but the professor was instead. And she did seem too fast for her age, which might mean that maybe it wasn't Macready at all, but perhaps Aslan guiding them to the wardrobe.