Chamber of Secrets

Ernie Macmillan is apparently one of the students who is helped after the grand chaos of the Duelling Club by Professor Lockhart: "Up you get, Macmillan..."

Identified only as Ernie, he is apparently the leader of the group of Hufflepuffs discussing Justin Finch-Fletchley's decision to remain in hiding when Harry Potter is thought to be the Heir of Slytherin, in The Duelling Club. He more or less acts as spokesman of the group when Harry confronts them – as a scion of an old wizarding family, he feels that the Heir of Slytherin would not attack him – and later accuses Harry of petrifying Justin when Justin is found in the hallway. (Professor McGonagall addresses him here as "Macmillan", giving us his last name.) Ernie is then given the task of wafting the petrified Nearly Headless Nick up to the Hospital Wing with a large fan.

Later, when Hermione Granger is Petrified, Ernie makes a point of apologizing to Harry for having thought him the Heir of Slytherin. Ernie feels that, Muggle-born or not, Hermione would not be a victim if Harry had anything to do with it.

Goblet of Fire

Harry sees Ernie in the campground at the Quidditch World Cup.

Ernie is the student who runs off to tell the Hufflepuff common room, most notably Cedric Diggory, that the delegations from Durmstrang and Beauxbatons will be arriving on the 30th of October.

Order of the Phoenix

On the Hogwarts Express, Ron reports that Ernie Macmillan is one of the two Hufflepuff prefects; Hannah Abbott is the other.

When Harry, Ron and Hermione are on the way to Herbology, Ernie runs up to them and tells Harry out loud, that he fully believes Harry and Dumbledore's claims about how Voldemort has returned. It cheers Harry to know that some students who are not weird believe him – previously, only Luna, apart from Harry's friends, had expressed belief in his and Dumbledore's statements.

Ernie is one of the original members of the Defence Association, or as it becomes known, Dumbledore's Army. He shows up at the initial meeting in the Hog's Head, but is initially reluctant to sign his name to the paper that Hermione is passing around. "After all, I am a prefect," he says, meaning, presumably, that he has to set a good example. Hermione points out that she's not going to leave this bit of parchment lying around, after which he signs, though still with some show of reluctance.

As a member of Dumbledore's Army, his work seems to be decent but not worthy of too much comment. He is, apparently, able to conjure a Patronus before the group is betrayed and broken up.

As the OWL exams approach, Ernie takes up the disquieting habit of asking everyone how much studying they are doing, and proudly saying that he is averaging eight hours a day.

Ernie is also one of the six members of Dumbledore's Army who see Draco Malfoy attempting to jinx Harry on the Hogwarts Express, and promptly return fire, leaving Draco, Crabbe, and Goyle looking like gigantic slugs squeezed into Hogwarts robes.

Half-Blood Prince

Ernie first appears in Professor Slughorn's NEWT-level Potions class, where it is revealed that he is the only Hufflepuff in their year who has managed to get in to it. When the trio come in he says hi to them and later sits at their table.

Deathly Hallows

Ernie is one of the group of Dumbledore's Army alumni who have retreated into the Room of Requirement, thus being able to stay on at the school and keep a form of guerrilla warfare going on against the Dark curriculum. With the others, he greets Harry's arrival with loud cheers and demands to know when they will be taking Hogwarts back.

Ernie takes part in the battle for Hogwarts. When Harry has to get to the Shrieking Shack, he finds his way to the Whomping Willow blocked by Dementors. He is unable to produce much of a Patronus himself, but Ernie, Luna Lovegood, and Seamus Finnigan arrive with their Patronuses and clear his way. The passage of a twenty-foot-tall giant separates them, and Harry, Ron, and Hermione proceed to the Shrieking Shack through the tunnel.