Mendelsohn, 54, has become one of Hollywood's favourite character actors since his Emmy-winning turn in Netflix drama "Bloodline" and appearances in blockbusters like "Rogue One" and "Ready Player One".
But it is his starring role as France's best-known designer in "The New Look", currently playing on Apple TV, that has been closest to his heart.
"The longer I was in those shoes, the more I loved him," said Mendelsohn.
He met AFP Tuesday at the Gallerie Dior in Paris, just after putting in an appearance at the label's latest fashion week show.
"The New Look" delves into Christian Dior's traumatic origins in Nazi-occupied Paris during World War II, as well as the story Gabriele "Coco" Chanel, played by Juliette Binoche.
"I do love him more than anyone I've ever played. In terms of a hero, he's sensitive, he's anxious, so full of doubt, full of self-loathing... but he still did so many impactful things."
Mendelsohn said he was attracted to the character's complex inner demons.
"He loathed himself because his public self was not close to his private self, and he couldn't reconcile the two," he said.
Mendelsohn sat with the show's costume designers for hours to immerse himself in their work.
"We shot it all here in Paris which was such a gift," he said. "There's something about this city and this culture."
The show is all in English, and Mendelsohn took on a French accent for the role: "An Australian accent for Christian Dior is not really gonna work!"
For all his immersion, he admitted he has "only scratched the surface" of what the industry is about.
"I come from a fairly typical Australian surburban male background, not exactly understanding a lot about fashion and how it interacts with the zeitgeist," he said.
"I wouldn't say I've become a fashionista since doing this show, but I'm certainly more appreciative of the role that fashion plays."
Looking dapper in a grey Dior suit, he added: "I've got to an age where I think I'm better off wearing suits rather than just jeans and a T-shirt!"
