Jez Guthrie's Ford Fiesta was followed by police on the A38 in Sutton at 7.35pm on December 19 last year, said prosecutor Fatima Laher.
When he was stopped, officers smelled cannabis and found a small amount of cannabis leaf.
A blood test revealed he had 4.2mcgs of cannabis in his system when the specified limit is 2mcgs.
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Simon Walton, mitigating, said there was no suggestion of bad driving and Guthrie smoked one joint before driving.
But he said his employers, the fire protection firm CRS, were ‘deeply upset’ and because he is going to be banned from driving he will also lose his job.
"It will also cause difficulties for his sister who works at the local hospital and uses him for transport," Mr Walton added.
“The consequences have been well and truly drilled home to him. It was extremely stupid.”
Guthrie, 28, of Wallis Road, Mansfield, admitted drug driving and possession of cannabis when he appeared at Mansfield Magistrates’ Court, on Thursday.
He was fined £120 and must pay £85 costs and a £34 surcharge. He was banned for 12 months.