Michael Sandford, 20, pleaded guilty in September to being an illegal alien in possession of a firearm and disrupting an official function.
He was accused of grabbing a policeman's gun at a rally in Las Vegas in order to shoot at the candidate.
His mother says "he is remorseful over what he did".
Sandford, of Dorking, Surrey, had faced up to 10 years in prison over the alleged assassination plot.
The court heard that he could be eligible for release in four months' time, at which point he will be deported to the UK.
The defence team said their client is autistic and suffers seizures and obsession-compulsion anxiety.
Michael Sandford appeared in court in orange prison garb, looking pale and slight, his ankles shackled.
He smiled as members of his family waved to him and mouthed: "I love you."
Sandford then broke down in tears as he apologised for what he had done and for taking up time and
costing the taxpayer money.
"I just feel terrible about it," he said.
The judge James Mahan appeared sympathetic, telling Sandford: "I don't think you harboured malice in your heart."
His mother, Lynne Sandford, also addressed the judge, saying her son was "cherished and adored".
"It breaks my heart to see him in this environment," she said, as she appealed for a lenient sentence on the basis that her son needed treatment and the support of his family.