The journalist and activist appeared in court on Wednesday after allegations that he made illegal recordings of animal cruelty which were later used in a TV report.
Mülln recorded the live plucking of geese in Hungary while investigating for an animal rights group in 2010 .
A whistleblower from the farm not only confirmed that much of their product goes to Germany, but secretly recorded a warehouse worker saying "that's live plucking, it's forbidden".
The report was broadcast in 2010, but Mülln has not faced legal consequences until now.
"Germany's legal machinery is very slow so it took them until 2014 to bring me to court, because a feather down company tried to sue me," Mülln told The Local.
But he's glad of the delay, as the court case has brought animal welfare back into the public eye.
"Court cases are always good. They bring publicity and fundamental new law decisions," he said.
Recording without permission can incur a €3,500 fine, but the judge applauded Mülln's hard work and determination after viewing the footage.
He was not charged on the condition that he pay €1,500 to an animal welfare organization. Mülln thought it a steep fee, as he considers himself innocent, but said he was happy the money went to a good cause.
The 34-year-old has been campaigning for animal rights for 21 years. His father was a meat trader and they lived in a rural area so Mülln became curious of what was behind the walls of the factory farms at a young age, he told The Local.
"I looked over the fence and saw some horrible, horrible things," he said.
As well as fighting against live plucking, Mülln has been closely involved with investigations into foie gras, the liver delicacy produced by force feeding ducks or geese.
He claims to be one of the very few people in the world that really knows about the business and "where they do their dirty stuff". This has landed him in some dangerous situations.
The foie gras industry "hate me like hell" Mülln said. He has had what he described as "murder threats from strange militia in Hungry". Once a man in a balaclava with an axe tried to attack him and another time somebody tried to knock him down with their car.
Mülln believes the court case has brought him some protection as someone under the media spotlight is less likely to be attacked, he said.
Mülln founded his own organisation, SOKO Tierschutz, because he wanted to start an organization that focuses on animals which "don't look nice but are in big need".
Louisa Lopez is interning at The Local Germany for the month of August.