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 university researcher sent 1,500 identical CVs to German firms - except that some bore the name Meryem Ozturk and others the name Sandra Bauer.
In 18.8% of cases Sandra Bauer was invited for interview, whereas the figure for Meryem was just 13.5%.
When the photo of Meryem showed her in a headscarf only 4.2% invited her.
The study was published by the Institute for the Study of Labour, in Bonn. The researcher was Doris Weichselbaumer from the University of Linz, in Austria.
The findings are especially significant in light of Germany's current efforts to integrate record numbers of Muslim migrants, many of them refugees from the conflicts in Syria, Iraq and Afghanistan.
More than a million asylum seekers reached Germany last year, and there has been a backlash from nationalist groups, especially Pegida and Alternative for Germany (AfD).
With an estimated three million people of Turkish origin, Germany is home to the largest Turkish diaspora.
In her study, Ms Weichselbaumer said that "a very modern binding of the headscarf was chosen to signal that the applicant was a young, modern woman who could easily fit into a secular environment".
It is normal in Germany to attach a photo to a job application, she pointed out.
The result of the experiment "implies that the candidate with the headscarf had to send 4.5 times as many applications as an identical applicant with a German name and no headscarf to receive the same number of callbacks for interview", she said.
Discrimination appeared to intensify when the job required a higher skill level. When Ms Ozturk in a headscarf was applying for a secretarial job she had to send off 3.5 times more applications than Ms Bauer. For the post of chief accountant Ms Ozturk had to make 7.6 times more applications.
Ms Weichselbaumer has found similar levels of discrimination in Austria. A female Nigerian job applicant was only half as successful as an Austrian woman in getting invitations to interview, her research showed in 2013.
German restaurant in Islamic veil row
Restaurant 'turns away Muslim women'

Police were called to Honeywell Drive, in Stepps, just after 21:00 on Sunday.
The man is said to be in a stable condition in hospital.
A Halfords customer's dashboard camera caught a mechanic speeding in his car while it was booked in for an MOT.
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An experiment in Germany has found evidence of job discrimination against women with Turkish names - and even more if they wear an Islamic headscarf.
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A 42-year-old man is being treated in hospital after being shot in a targeted attack near Glasgow.