Shaun Ingram from Saltash booked his Ford Focus ST in for a major service at Halfords Autocentre in Plymouth in March.
On its return he discovered the dashboard camera recorded footage of his car being driven 57 mph in a 30 mph zone.
Halfords said the mechanic was a temporary employee who had been let go.
After the complaint the company has apologised and offered to refund the Â£255 for the MOT and service.
Mr Ingram said the camera was in plain sight on the windscreen and was not left on to "catch anybody out" during the service.
The camera recorded almost seven hours of footage including the mechanic swearing and talking about test-driving the car.
Mr Ingram said: "When I saw the footage I was left open-mouthed. The mechanic was abusing my car and my trust.
"As a nationwide company I thought I had nothing to worry about. I feel very let down."
The regional manager for Halfords Autocentres, Daren Stone, told Mr Ingram that the mechanic's behaviour was unacceptable.
He said: "He drove your car in what I can only describe as an appalling manner.
"I do not and will never condone that level of pure neglect which is why the mechanic in question has been removed from working within the Halfords Autocentre Group."
Mr Ingram said: "From now on I will always wait by the car whilst it is being serviced."
A spokesperson for Halfords head office said: "We are looking into the full details of the incident and will deal with this issue internally as a matter of urgency to ensure this type of incident does not occur again in the future."
A Halfords customer's dashboard camera caught a mechanic speeding in his car while it was booked in for an MOT.