Amos and his son, Cedric, are waiting on the top of Stoatshead Hill when the Weasleys, Harry and Hermione arrive. He calls the group over, having found the Portkey, an old boot. Amos and Cedric had to get up at 2 o'clock in the morning to get there in time. He tells Arthur he will be glad when Cedric passes his Apparition test, but is just happy to be going to the Quidditch World Cup.
Amos is intrigued to meet Harry Potter, still thrilled that Cedric beat him in a Quidditch match the previous year, and says that it is something Cedric can tell his grandchildren. He is a good-natured man, but is convinced that his son is being too modest, and that Harry will admit the best man won in the end.
Amos holds out the old boot a minute before the Portkey is due to depart. He, Arthur and Cedric are the only ones of the group who remain on their feet after the Portkey delivers them to the campsite.
Amos Diggory arrives in the clearing in the woods after the Dark Mark has been conjured. He is certain that the culprit can't have escaped all the Stunning Charms sent by the Ministry wizards, and marches across the clearing to where the incantation for the Dark Mark was shouted. He is stunned when he finds Winky, Barty Crouch's house-elf, unconscious in that area. He tells Arthur that Winky was found with a wand in her hand, and is positive she was the one who conjured the Mark.
When Winky is revived, Amos asks her to explain why she was discovered there, and how she came to be in possession of a wand. He is astonished when Harry tells them that the wand is his, and for a brief moment is convinced that Harry is confessing to the crime. He accuses Winky once more of conjuring the Mark; taking out his own wand, he places it tip to tip against Harry's wand, and casts Priori Incantato. He eradicates the shadow of the Dark Mark that erupts where the two wands meet, triumphant that his theory has been proved correct. Amos is horrified when he realises that, by accusing Winky, he has inadvertently implicated Barty Crouch in the affair, as the elf would have had to have learnt the spell from her master. He is unimpressed when Winky tells them that she found the wand in the woods, but didn't see anyone else in the area. Amos reluctantly allows Crouch to deal with Winky's punishment.
Amos calls on Arthur Weasley via the Floo Network to tell him of an incident at Mad-Eye Moody's house, and to ask him to go there immediately. He is worried that the Improper Use of Magic officials will try to prosecute Mad-Eye, and wants to try to get him charged by Arthur's department on a less serious charge.
Amos and his wife wait in the chamber off the Great Hall on the morning of the third task, along with the other families of the Triwizard champions. When Harry arrives in the room, Amos looks him up and down, and suggests that Harry isn't feeling quite as full of himself now that Cedric has caught him up on points. Cedric tells Harry that Amos has been angry ever since Rita Skeeter's article about the Triwizard Tournament was published, implying that Harry was the only Hogwarts champion. Amos is annoyed that Harry made no attempt to correct Skeeter, but is confident that Cedric will beat Harry, as he did in the Quidditch match the previous year.
Amos Diggory runs towards Harry when the boy is returned to Hogwarts by the Triwizard Cup, after hearing the rumours that his son Cedric is dead. He is intercepted by Professor Dumbledore before he sees the body of his son.
Amos and his wife spend time with Professor Sprout immediately after the third task, as she is the Head of Hufflepuff house, and knew Cedric best. They are joined by Professor Dumbledore later in the evening.
Amos and his wife meet with Harry the day after the third task. The Diggorys do not blame Harry for what happened to their son, and thank him for returning Cedric's body to them. Amos sobs throughout most of the interview. When Harry tells them how Cedric died, Amos is reminded by his wife that Cedric died just after winning the Tournament, both comforted that their son must have been happy and that he didn't suffer. The Diggorys refuse to take the Triwizard winnings from Harry, insisting that he keep them.