Raphael Geys, former managing director of European fixed income sales, was let go by the bank in November 2007 without reason and offered Â£7 million as severance pay.
The banker launched proceedings against the institution, claiming that he was owed more than Â£11 million in compensation.
Under his tenure, the department doubled profits from Â£183 million to Â£394 million.
Documents presented to the court by Mr Geysâ legal team alleged that he was sacked for being âtoo successfulâ because the financial rewards in his pay package were âtoo generousâ.
Tom Custance, head of dispute resolution at Fox Williams LLP, acting on behalf of Mr Geys, said: "We are obviously very pleased with the outcome of this case which entirely vindicates Mr Geys's decision to pursue SociÃ©tÃ© GÃ©nÃ©rale for the payments properly due to him on termination of his employment."
The final compensation sum is expected to be decided upon at a future hearing.
SociÃ©tÃ© GÃ©nÃ©rale said it would launch an appeal against the decision.
By Jim Ottewill