Ken Brown was escorting the vehicle on foot when it struck him at the Windscale site in Seascale on 5 May 2011.
His employer, Johnson Controls Ltd, was prosecuted by the Health & Safety Executive (HSE) after an investigation found there had not been a safe system of work for the task, and relevant training had not been provided.
Carlisle Crown Court heard how the 62-year-old from Distington had been standing close to the front of the telescopic boom access platform to direct it along a one-way road at the site when it ran over his left leg.
Mr Brown was taken to hospital where doctors had to amputate his leg above the knee.
The HSE investigation found that Mr Brown and other staff at Johnson Controls had escorted boomlifts on foot several times a month, for at least 14 months, prior to him being injured.
Johnson Controls Ltd, part of a global organisation that employs more than 160,000 people, pleaded guilty to breaching Section 2(1) of the Health and Safety at Work etc Act 1974 by failing to ensure the safety of employees.
The company, of Waterberry Drive, Waterlooville, Hampshire, was fined £65,000 and ordered to pay £8,162 in prosecution costs on 8 October 2012.
Speaking after the hearing, HSE investigating inspector 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."