Lynden Scourfield, a former manager with HBOS, pleaded guilty to six counts including corruption.
Five other defendants, including so-called turnaround consultants, were also convicted.
In exchange for bribes, Scourfield told customers to use the turnaround firm.
Mark Dobson, who was also a manager at HBOS, David Mills, Michael Bancroft, Alison Mills, and John Anthony Cartwright were convicted at Southwark Crown Court on counts including bribery, fraud and money laundering.
One other defendant, Jonathan Cohen, was acquitted.
Scourfield had been convicted after pleading guilty at an earlier trial last year.
The five newly convicted people will be sentenced on Thursday.
The CPS special prosecutor, Stephen Rowland, said the case was one of the largest and most complex the special fraud division had ever prosecuted.
"It involved millions of documents, a lot of the material we had to look at was electronic and of course in this day and age the capacity for electronic media is huge," he said.
"So we had a very large amount of material to work through and to consider."
Businessmen Bancroft and Mills arranged sex parties, exotic foreign holidays, cash in brown envelopes and other favours for Scourfield between 2003 and 2007.
In exchange for the bribes, Scourfield would require the bank's small business customers to use the firm of consultants run by Mills and his wife Alison, Quayside Corporate Services.
Quayside purported to be turnaround consultants, offering business experience and expertise to help small business customers improve their fortunes.
But far from helping turn businesses around, Mills and his associates were milking them for huge fees and using their relationship with the bank to bully the business owners and strip them of their assets.
In cash fees alone, according to prosecutors in the trial, £28m went through the accounts of Mills, his wife and their associated companies.