The Western Isles MP had been on trial on a charge of dangerous driving, but was instead convicted of the alternative charge.
It comes at the end of a trial at Lochmaddy Sheriff Court on the island of Uist in the Outer Hebrides.
As well as being banned from driving for three months he has been fined £1,500 by Sheriff Gordon Lamont.
At earlier proceedings in March, the court was told that MacNeil, MP for the Western Isles constituency Na h-Eileanan an Iar, was involved in a two-vehicle collision on Barra in October 2020.
Motorcyclist Cailean MacNeil, who was 17 at the time, suffered multiple leg fractures and had to be airlifted to Glasgow for treatment.
MacNeill denied performing a manoeuvre without keeping a proper lookout, and causing serious injury .
A witness told the court that he thought Cailean MacNeil had been travelling at about 30mph in a 20mph area, which the victim denied..
He had been riding a dirt bike along the A888, near the Castlebay Community School on MacNeil’s home island of Barra, when the politician pulled out and crashed into him.
At the time a Police Scotland spokesman said: “About 7.08pm on Friday, 30 October, 2020, a two vehicle road traffic crash happened on the A888 near Castlebay Community School, Isle of Barra.
“A Ford Focus car was in collision with a motorcycle.
"The 17-year-old male rider of the motorcycle suffered a non-life threatening injury and was taken to hospital. Nobody else was injured.”