Chamber of Secrets

When Dobby performs a Hover Charm in the Dursley's house, Harry is believed to be the one actually doing magic. The warning letter that arrives is signed by Mafalda Hopkirk.

Order of the Phoenix

Harry again does magic, this time using the Patronus charm to repel Dementors who are attacking himself and his cousin Dudley. The letter of reprimand that is sent to him by Owl Post is signed by Mafalda Hopkirk, and so is the following letter rescinding the immediate punishment and setting a hearing date.

Deathly Hallows

When Harry, Ron, and Hermione determine that they must get into the Ministry to search for the locket Horcrux which has been appropriated by Dolores Umbridge, they disguise themselves as Ministry employees, using Polyjuice potion to do so. Hermione ends up disguised as Mafalda Hopkirk; the real Mafalda Hopkirk is left, Stunned, behind a closed door.

Once in the Ministry, Harry and Hermione take the elevator to the top floor, where they understand Umbridge's office is. Umbridge is at the elevator and, recognizing Hopkirk, assumes she was sent by Travers. The two of them return through the Atrium to the courtrooms, where "Mafalda" is to act as recorder for the day's session of the Muggle-Born Registration Commission. Very shortly after Harry re-joins her, they depart the Ministry and Hermione's disguise wears off.