Kevin Steele, 51, from Kent, who used to work at solicitors Mishcon de Reya (MDR), was convicted of forgery and two fraud offences at Southwark Crown Court.
Two other defendants Michael Shephard, 50, and Mark Pattinson, 38, both of Lancashire, pleaded guilty before the trial to one count each of forgery and fraud.
Steele was found to have helped former client Shephard and his friend Pattinson fraudulently access a loan from Zurich-based EFG Private Bank in 2008.
With the help of fake bank letters they falsely claimed £76.4m held in two accounts at Bank Julius & Co in Guernsey could be used as security for a €22 million loan.
And Steele was also convicted of abusing his position as partner in such a way as would expose MDR to risk of loss.
Commenting on the outcome, SFO director Richard Alderman said: “I welcome today's verdict. It's a victory for honesty and fair dealing in business.”
The men are all due to be sentenced on 9th January 2012. MDR famously had previously represented Diana during her divorce from Prince Charles.
By Andy Pearce