That cloak of secrecy it wrapped around itself was ripped apart on Sunday when media organizations around the world publishing information from a massive leak from the firm's supposedly secure data center.
Politicians, sports stars, celebrities -- many were named in the 11 million pages of documents, according to information starting to be released by the International Consortium of Investigating Journalists (ICIJ), which is parsing the data.
The revelation detailing the off-shore structures of many wealthy clients is a "crime" and an "attack" on Panama, the law firm maintains.
"This is a crime, a felony," Ramon Fonseca, one of the founders of Mossack Fonseca, told AFP.
Also Read
"This is an attack on Panama because certain countries don't like it that we are so competitive in attracting companies," he said.
So who runs Mossack Fonseca, whose headquarters are housed in a fairly non-descript mirrored building in Panama's business district?
Mossack's father was a Nazi in World War II, serving in Hitler's Waffen-SS, according to the ICIJ, citing US Army records. It said "old intelligence files" showed the father had offered to spy for the CIA.
The other founder is Fonseca, born in 1952. He, too, got his law degree in Panama but also studied at the prestigious London School of Economics, and once said in an interview he had mulled becoming a priest.
The ICIJ said the leak shows that half of the companies the law firm incorporated -- more than 113,000 -- were done so in that fiscal paradise.
But Mossack Fonseca also branched out to the Pacific, to a tiny island nation called Niue.
(REOPENS FGN 20)
When the British Virgin Islands was forced to clamp down on some methods that had previously permitted anonymous ownership of companies, Mossack Fonseca moved business to Panama and to the Caribbean island of Anguilla.
But other countries took an increased interest in what it was doing. In Brazil, it was named as being one of the parties within a huge bribery scandal unfolding involving the state oil company Petrobras.
It also came under scrutiny in the US state of Nevada, where a judge determined that it had willfully tried to cover up its management role over its local branch there.
Last month, Fonseca -- who had also been an advisor to Panamanian President Juan Carlos Varela since 2014 -- declared he was taking a leave of absence.