North Antrim Magistrates Court heard how a man purchased a Hyndman's malt loaf from a supermarket in the Ballymoney area before Christmas 2007.
When he unwrapped the loaf he discovered the small lifeless mammal embedded in the base of the bread.
The judge fined the company, D Hyndman and Son Ltd, Maghera, £1,000 plus costs for placing unsafe food on the market.
The managing director of the company was present in court to answer the charge.
A defense lawyer told the court that the presence of the mouse was a shock to the company. He suggested it might have been put in the tin to "sabotage" the baker who has been in business for 60 years and has never had any complaints.
The judge was shown pictures of the mouse in the loaf. The court heard that tins in which the bread is baked were oiled the night before they were filled with dough.