After an 11-week trial, 50-year-old former senior derivatives trader Philippe Moryoussef was found guilty of conspiracy to defraud by dishonestly manipulating Euribor (Euro interbank offered rate) for profit between January 2005 and December 2009.
Three of his co-defendants, however, face a possible retrial.
The prosecution was the first to focus on Euribor, a reference for rates on more than $150 trillion (113.52 trillion pounds)of financial contracts and consumer loans worldwide.
Moroccan-born Moryoussef is expected to be sentenced on July 20 alongside compatriot Christian Bittar, a 46-year-old Senegal-born former Deutsche Bank trader who pleaded guilty in March before the trial began. He is already in custody.
Bittar, described by investigators as one of the world's best-paid traders, earned about 60 million pounds ($80 million) over the indictment period alone.
A conspiracy to defraud conviction carries a jail sentence of up to 10 years.
Reporting restrictions were lifted on Thursday after the jury failed to reach a verdict on three of Moryoussef's former colleagues; Italian-born Carlo Palombo, a former trader who reported to Moryoussef, Sisse Bohart, a Danish ex-junior trader and rate submitter, and her one-time boss Colin Bermingham.
Achim Kraemer, a 22-year Deutsche Bank veteran still employed by the bank, was acquitted unanimously. He said in a statement that he wanted to rebuild his life and resume his career.
"(Bittar and Moryoussef) were senior figures who abused their positions for personal gain and to advantage the banks they worked for," said Mark Thompson, interim director of Britain's Serious Fraud Office (SFO).
The SFO said it would decide within seven days whether to seek a retrial of Palombo, Bermingham and Bohart.
Deutsche Bank and Barclays declined to comment
Global authorities have fined leading banks and brokerages about $9 billion and charged about 30 people over rate-rigging allegations. Barclays paid $453 million in 2012 and Deutsche Bank settled its case for $2.5 billion in 2015.
The SFO's fifth rate-rigging trial brings to seven the number of people convicted in a six-year investigation. But nine have been acquitted and the agency also faces the challenge of enforcing its latest conviction.
Moryoussef left for France after Bittar's guilty plea was made public and had no defending counsel in court. His Paris-based lawyer, Francois De Casto, said his client was under the protection of French law.
"Philippe Moryoussef will eventually refer this case to the European Court of Human Rights," he said.
The SFO had orginally wanted to prosecute 11 individuals in the case, but French and German authorities refused to extradite five of their citizens.
Prosecutors cast Bittar and Moryoussef, old friends who cut their teeth in banking at France's Societe Generale, as the ringleaders of an international, inter-bank scam to deliberately flout rules by asking rate submitters to nudge rates up or down to bolster trading books in a "zero sum game".
Vast derivatives profits could ride on a basis point, or 0.01 percent, change in Euribor rates, which are set over various time frames.
In messages read out in court, Moryoussef emailed Palombo on October 1, 2006, saying: "My mobile won't be connected on Monday. Two things, ask Sisse to put a high 6 month (Euribor)... and Christian as well ... we have 2 billion on that date. We definitely have to make one BP (basis point) on that fixing. Good luck, bye."
The defendants denied dishonesty, said they had learnt their trade on the job and that they believed there was nothing wrong with requesting or accommodating preferential trades.
Rate requests were made openly and fell within a range of equally valid rates, they said.
Defence lawyers told the court there was no evidence that Kraemer, Palombo, Bohart and Bermingham had financially benefited from the alleged scam.
In a statement read out to the jury during the trial, Moryoussef said trader requests were an established practice at Barclays when he joined in 2005 and that he believed submitters would choose a correct rate from a permitted range of estimates.
(Additional reporting by Emma Rumney; Editing by Silvia Aloisi, Keith Weir and David Goodman)
By Kirstin Ridley