After entering Hagrid's house, Macnair asks where the 'beast' is. He looks out of Hagrid's window and sees the Hippogriff standing in the pumpkin patch. He turns back to face the room and signs the official notice of execution alongside Hagrid. Macnair is angry and confused when they leave the hut to find Buckbeak gone, as he had seen the Hippogriff tethered to the fence just moments before. He swings his axe into the fence in frustration, and tells the Committee that they should begin a search of the grounds and the Forest. He walks back into the house after Professor Dumbledore suggests the Hippogriff has flown away, cursing softly.
Walden Macnair is one of the Death Eaters who appears in the Department of Mysteries. During the battle, he is Stunned by Harry; as he falls to the ground, Macnair's mask slips off, revealing his identity.