Ken Brown was escorting the vehicle on foot when it struck him at the Windscale site on May 5 2011.
His employer, Johnson Controls Ltd, was prosecuted following a Health and Safety Executive investigation.
Carlisle Crown Court heard the 62-year-old from Distington had been standing close to the front of the cherry picker to direct it along a one-way road at the site when it ran over his left leg.
Brown was taken to hospital where doctors had to amputate his leg above the knee.
The HSE investigation found that Brown and other staff at Johnson Controls had escorted cherry pickers on foot several times a month, for at least 14 months, prior to him being injured.
The only advice the company gave to its employees when directing cherry pickers was to wear a high visibility waistcoat. No specific training was provided for the task.
Johnson Controls pleaded guilty to safety breaches and was fined £65,000 and ordered to pay £8,162 in prosecution costs.
Speaking after the hearing, the investigating inspector at HSE, Faye Wingfield, said: “Kenneth Brown has suffered a terrible injury that will affect him for the rest of his life due to failings of his employer.
“Vehicles continue to be a major cause of serious injuries in the workplace, and the first principle of any employer should be to keep people and vehicles apart.
“It is questionable whether Johnson Controls actually needed a member of staff on foot to direct the cherry picker, given that it was travelling forwards along a road in a one-way system.
“But if the risk assessment decided someone was needed to escort the vehicle then a safe system of work needed to be devised. Employees should also have been given appropriate training, including how to communicate effectively with the driver.”