Michael Sandford, 20, from Dorking, Surrey, was accused of trying to grab a police officer's gun to shoot Mr Trump at a Las Vegas rally on 18 June.
He pleaded guilty to being an illegal alien in possession of a firearm and disrupting an official function.
Lynne Sandford said she feared Mr Trump would "make an example" of her son.
Sandford had previously pleaded not guilty to charges of disrupting government business and official functions, being an illegal alien, and possession of a gun.
He had been due to stand trial before he signed a plea agreement at a hearing in September.
He is due to be sentenced on Tuesday in Las Vegas.
Sandford's lawyers said he suffers seizures, obsession-compulsion anxiety and autism spectrum disorders.
Mrs Sandford said: "I'm apprehensive and nervous not knowing what the sentence might be.
"Michael is terrified. He phoned me 15 times through the night with severe anxiety over what the judge is going to say to him and the sentence he will be given," she said.
"Although he signed a plea agreement, which should mean he receives between 18 and 24 months, the judge is able to overrule that.
"Michael is afraid that because Donald Trump is now president-elect, it may have some bearing on the judge's sentencing."
Sandford may have faced up to 20 years in a US prison if he had been convicted at trial.
Mrs Sandford said: "Michael is remorseful over what he did. He fears Donald Trump may intervene and choose to make an example of him."
The mother of a British man accused of trying to shoot Donald Trump says he is "terrified" of a lengthy prison sentence.