The solicitor general was sentenced at North Wiltshire magistrates court in her absence following the incident on the M4 motorway last month.
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Ms Harman's speed was recorded as 99mph in a 70mph zone
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Attorney General's office spokeswoman
Ms Harman had already made it clear she would plead guilty at the first available opportunity.
At the time the MP was stopped her son was travelling in the car with her.
She was taking him back to a university in Bristol after the Christmas holidays.
Solicitor Tim Dixon, who was representing the solicitor general in her absence, said: "Ms Harman is of course extremely sorry that she exceeded the speed limit and she does not seek to make any excuse for the offence.
"She was travelling in the outside lane when she approached and overtook a gaggle of HGV vehicles, she thought she was adhering to the speed limit."
Ms Harman had driven for 26 years without incident and had a full, clean driving licence, Mr Dixon added.
'No comment'
A Downing Street spokesman said in January: "Harriet Harman said she is sorry and made clear she intends to plead guilty."
After the verdict the Attorney General's office - of which Ms Harman role is part - said they had no comment on her conviction.
The solicitor general's role is as deputy chief legal adviser to the government.
Ms Harman also superintends the Crown Prosecution Service.