Stipendiary magistrate Tim Workman ruled that the company could not be held responsible for what had happened because he believed the mouse parts were among hazelnuts which had been sent to the company's main factory in Britain from a supplier in Turkey.
Mars, he said, had been diligent in checking the Turkish operation so could not be found guilty of negligence under the Act.
The case at Horseferry Road magistrates' court in south-west London was brought by Westminster Council following a complaint by office worker Marie Henriques, 32.
Ms Henriques told the court how she had been physically sick after finding a "grey furry object" as she finished a Topic bar which had been boughtfrom a confectioner in Piccadilly in April 1997.
The object was later found to be part of a mouse.Reuse content