Michael Sandford, 20, from Dorking, Surrey, was due to stand trial over an incident at a Trump rally in Las Vegas on 18 June, the BBC reported.
He pleaded guilty to being an illegal alien in possession of a firearm and disrupting an official function.
His plea means he could be deported and his sentence reduced.
Sandford may have faced up to 20 years in a US prison if he had been convicted at trial of both charges.
He was due to stand trial later in September over an incident at a rally in which he allegedly tried to grab a policeman’s gun to attack the Republican presidential candidate.