Half-Blood Prince

On the Hogwarts Express, Professor Slughorn invites a number of students to his compartment for lunch, including Harry Potter, Neville Longbottom, Ginny Weasley, Marcus Belby, Blaise Zabini, and Cormac McLaggen. It seems that most of them are invited because of their famous relatives, or because they are famous in their own right as Harry is, or because of their own apparent magical ability: Ginny is invited because Slughorn saw her performing a flawless Bat-Bogey hex. Harry recognizes this as Slughorn re-establishing his Slug Club.

McLaggen tries out for the Gryffindor Quidditch team, hoping to replace Ron Weasley as Keeper. Each Keeper candidate has five chances to stop shots on goal; Ron stops all five, but McLaggen fumbles the final one, and Ron is selected as Keeper. Ron comments that McLaggen had looked Confunded on his last shot; Hermione blushes, but Ron doesn't notice. Later, Harry sees McLaggen apparently having trouble negotiating the front steps into Hogwarts, and asks Hermione about it. Hermione, aware of his bad temper and denigrating comments about Ron and the other Keepers being tried, quietly admits having Confunded him during his trials so that Harry would have a valid reason to select Ron instead of him.

With Ron thoroughly entangled in Lavender Brown, Hermione does not have anyone she can take to Professor Slughorn's "Slug Club" Christmas party. She invites McLaggen to make Ron jealous. Once there, finding McLaggen's unrelenting efforts to maneuver her under the mistletoe tiresome, she manages to lose him, instead talking to Harry and his date, Luna Lovegood.

When Ron is put out of action by a glass of poisoned mead, Harry reluctantly puts McLaggen in the team as Keeper. McLaggen immediately starts acting like team captain, trying to set tactics for the games, and telling the other players what is wrong with what they are doing. During the game against Hufflepuff, he borrows a Beater's bat to demonstrate the proper way to hit a Bludger, mis-hits it, and fractures Harry's skull. With Harry out of the game, Gryffindor loses very heavily to Hufflepuff. McLaggen is, of course, cut from the team immediately after Harry regains consciousness.