It was Ludo who managed to get the Weasley family such good tickets for the Quidditch World Cup final after Arthur helped his brother, Otto, out of a spot of trouble. Percy tells Arthur that he thinks Ludo has mishandled the disappearance of Bertha Jorkins, a woman who works in Bagman's department, The Department of Magical Games and Sports. According to Mr Weasley, Ludo says that Bertha has got lost many times before, and therefore isn't worried about her disappearance. He believes that Bertha probably misread the map and ended up in Australia instead of Albania.
According to a Ministry wizard, Ludo has been causing them some trouble, walking around talking about Bludgers and Quaffles with no concern about anti-Muggle security. Mr Weasley says that Ludo has always been a bit lax about security. He is an extremely enthusiastic Head of the Sports Department at the Ministry, and previously played Quidditch for England. He was the best Beater the Wimborne Wasps Quidditch team had ever had.
Ludo gives the welcome speech at the Quidditch World Cup, and introduces the team mascots as they enter the stadium. Ludo announces the Bulgarian and Irish Quidditch players as they fly onto the pitch from their respective entrances, and the referee, Hassan Mostafa. He commentates throughout the match, using the incantation Sonorus to amplify his voice to the whole stadium.
Ludo is shocked by the result of the match, and hoarsely tells those in the Top Box that people will be talking about it for years. As he talks, he is confronted by Fred and George, who stand in front of him, grinning, ready to collect their winnings from him.
Harry, Ron and Hermione come across Ludo in the woods. His face is white and strained, and he is confused as to why the three of them are there. When he hears that there is a riot going on at the campsite he swears, looking distracted, before Disapparating on the spot.
Ludo Apparates into the clearing in the woods after Winky the house-elf has been found. He is shocked when he realises that the Ministry wizards think Winky is the one who conjured the Dark Mark, and is certain that she wouldn't know how to cast such a spell.
Ludo Bagman visits Hogwarts on the same day the Beauxbatons and Durmstrang delegations arrive. He is one of the five judges of the Triwizard Tournament, alongside Madame Maxime, Professor Karkaroff, Professor Dumbledore and Barty Crouch. Ludo is introduced to the students in the Great Hall by Professor Dumbledore; when his name is called he acknowledges the loud applause with a jovial wave of his hand.
Ludo awaits the announcement of the three Triwizard champions after the Hallowe'en feast. He beams and winks at various students as the food is cleared away.
Ludo is astonished when Harry's name is produced by the Goblet of Fire, and follows the boy into the room off the Great Hall. He introduces Harry to the other three champions; despite the unusual circumstances, Ludo is bewildered when the champions do not initially believe him. He tells the champions that, as the age restriction was only imposed that year as an extra safety measure, he doesn't think Harry will be able to pull out of the Tournament. Ludo defers to Barty Crouch's knowledge of the Triwizard Tournament rules, and accepts that as Harry's name came out of the Goblet of Fire, he must compete. He tries to explain to Professor Karkaroff and Madame Maxime that there is no way of re-igniting the Goblet of Fire until the next Triwizard Tournament. Ludo is agitated by Professor Moody's claim that someone is trying to harm Harry by entering him into the Tournament.
Bagman is present for the Wand Weighing ceremony; he sits in a chair behind the velvet-covered desk, talking to Rita Skeeter. He spots Harry when he enters the classroom, and bounds over to greet him. He explains what is going to happen during the afternoon, and introduces Harry to Rita. He allows Rita to guide Harry away for an interview. Bagman joins the other judges at their table for the official ceremony.
Ludo Bagman stands in the champions' tent before the first task. He tells the champions that their task is to collect the golden egg, and produces a purple bag from which they must choose their dragon. He tells Cedric to go towards to enclosure, ready to begin, and turns to Harry with a fatherly expression on his face. Bagman tells Harry he would be happy to share a few pointers if Harry wants, as he feels Harry is the underdog in the competition. He winks as he tells Harry that no one would find out if he helped Harry, but his assistance is declined. Bagman starts in alarm when he hears a whistle blow in the distance, and hurries off towards the judges seats. Bagman commentates during the first task, although he doesn't verbalise the final scores given by the judges. He is shocked when Harry manages to summon his Firebolt, astounded that Viktor Krum didn't think of the idea, and is further surprised when Harry turns out to be the quickest champion to reach his egg. After the first task, Bagman addresses the champions in the tent. He tells them that their golden egg contains a clue to the second task.
Ludo attends the Yule Ball wearing robes of bright purple with large yellow stars. He claps enthusiastically as the champions walk towards the top table before dinner. When the Weird Sisters begin their performance after dinner, Bagman enjoys a dance with Professor McGonagall. He is accosted by Fred and George as he makes his way back through the crowds but shakes them off fairly quickly, making his way over to Harry and Percy. He tells Percy that the twins were telling him about their fake wands, and were asking for advice on how to market their products.
Ludo Bagman is sitting in a shadowy corner of the Three Broomsticks pub with a group of goblins. He looks strained as he sits with the menacing creatures. Bagman spots Harry standing at the bar with Hermione and Ron, and hurries through the pub towards him. He tells Harry he has been hoping to run into him, and asks to speak to him alone. Bagman congratulates Harry for his performance against the Hungarian Horntail, but doesn't seem to be in any rush to get to the point of their conversation. He tells Harry that the goblins he is with are looking for Barty Crouch. Bagman asks Harry how he is getting on with figuring out the clue in the golden egg, and offers to help Harry if he needs a prod in the right direction. Bagman tells Harry that they all want a Hogwarts victory, but frowns when asked whether he has offered Cedric Diggory the same assistance. He looks affronted when Harry refuses his help but is unable to say much more as Fred and George approach the pair. Bagman declines the twins' offer of a drink and leaves the Three Broomsticks after one last disappointed glance at Harry.
Ludo asks if Harry knows what he is going to do in the second task before he allows it to begin. He officially starts the task by blowing his whistle, and watches the champions enter the lake.
When all the champions have arrived back safely, Bagman reveals the scores. He gives Professor Karkaroff a nasty look as he reveals that most of the judges found Harry's performance in the lake evidence of moral fibre, and worthy of full marks, but that they are awarding him forty-five.
Ludo Bagman comes up in the conversation between Harry, Ron, Hermione and Sirius about who could have stolen Harry's wand at the Quidditch World Cup. Sirius suggests that the reason Bagman didn't hurry to look for Bertha Jorkins is because she is a liability at the Ministry of Magic.
Ludo Bagman meets the four champions in the centre of the Hogwarts Quidditch pitch. He tells the champions that the Triwizard Cup will be hidden in the centre of a maze, and that the first champion to touch it will receive full marks for the third task. He tells the champions the order in which they will enter the maze, then starts to make his way back to the castle. He looks a little perturbed when Krum asks to walk with Harry, and offers to wait for Harry but is declined.
In one of the Pensieve memories, Ludo Bagman is revealed to have been on trial for passing information to Death Eaters during the First Wizarding War. At the time, Ludo was at the height of his Quidditch-playing fitness. He looks nervous as he sits before the Council of Magical Law, but glances around at the watching crowd, waving at a few of them. During the trial, he claims to have had no idea that the man he was associated with, Augustus Rookwood, was a Death Eater who was passing information on to Lord Voldemort. Bagman claims that Rookwood offered to get him a job at the Ministry once his Quidditch career ended, and that he thought the information he was passing on was being used to help the Ministry of Magic. The jury unanimously voted to acquit Bagman, much to the displeasure of Mr Crouch.
Bagman escorts the four champions to the Quidditch pitch after the evening feast to prepare for the third task. He asks Harry whether he is feeling alright as they walk, and hopes the boy is feeling confident. Bagman magically amplifies his voice to announce the third task to the spectators, and reveals the current standing of each of the champions. He gives a short blast of his whistle to begin the task, allowing the champions to enter the maze in their correct order.
Fred and George reveal that Ludo Bagman is the person they have been attempting to blackmail all year. Bagman paid the twins the money they won from him at the Quidditch World Cup in leprechaun gold, which disappeared after a few hours. Initially the twins thought that he had made a mistake, but Bagman ignored their letter, and made excuses to avoid them at Hogwarts. The twins discover that Bagman is in big trouble with the goblins, as he borrowed a lot of gold from them. A gang of them cornered him in the woods after the Quidditch World Cup and took all the gold he had on him, but it wasn't enough to cover all his debts. He has lost everything gambling, and hasn't got two Galleons to rub together. He attempted to pay the goblins back by placing a huge bet on Harry to win the Triwizard Tournament, but as Harry drew with Cedric Diggory he didn't win. Bagman made a run for it immediately after the third task.