Deborah Wale, 19, allegedly put the rodent in her employer's lunch after finding it in the loft of the nursing home where she worked in in Lymington, Hampshire.
But Miss Wale, of New Milton, Hampshire, claimed she had suffered sexual harassment and victimisation.
She said her employer, Pankaj Popat, a businessman based in London, had subjected her to a stream of lewd telephone calls and groped her.
She said Mr Popat would pat her bottom and on one occasion she alleged that he had come up behind her and groped her breasts as she lifted blankets from a cupboard.
Miss Wale told the tribunal: 'He was lecherous. He would phone me at night and tell me, 'I can see your underwear'.'
Her claim of sexual discrimination and victimisation was unanimously rejected by the tribunal sitting in Southampton.
Mr Popat told the tribunal that the mouse incident happened while he was on a visit to the Birchy Hill Nursing Home which he owns. He said he and the home's matron were eating sandwiches with one of the residents when they broke off to inspect some building work outside.
'We returned inside and I saw the dead mouse between two slices of bread. I was horrified,' he said. 'It could have been poisoned and the resident, who suffers from dementia, could have eaten it.
Miss Wale told the hearing that she found the mouse while she and two colleagues were searching for Christmas decorations in the attic. But she claimed she put it on a trolley to be disposed of and never saw it again.
The next day the matron telephoned her to say she was not wanted any more because of the incident.
Sultan Sutherland, a former colleague, said she had told Miss Wale to get rid of the mouse, and Miss Wale had taken it downstairs. When she came back, she said she had put it in a sandwich in the office.
'I then went down to check and I found the mouse in the sandwich. A resident was sitting staring at it,' said Miss Sutherland.
After the hearing Miss Wale said: 'I know I'm in the right and this has not put me off nursing.'