The Independent Parliamentary Standards Authority (IPSA) found that Mr Shannon was the highest claimant for staff constituency mileage last year.
But it said there had been no fraud.
It said “a number of errors” had been made by Mr Shannon “leading to claims for constituency office mileage that would not be allowable”.
The Strangford MP’s claims were five times greater than the second highest claiming MP and 37 times greater than the average claim by all MPs.
He accounted for 26.1% of all staff constituency mileage claimed by the entire House of Commons.
IPSA found that Mr Shannon’s staff had made mistakes in what they claimed for, which included delivering food to constituents from food banks.
The report found that Mr Shannon had taken on too much work and said he “still regards himself as a local councillor” and “needs to let go”.
Mr Shannon said that because the work MPs do is publicly funded there “must be no question of any abuse or dishonesty” in how those funds are used.
“I am particularly glad that nothing of this sort has happened in my case and that my integrity and that of my office has been upheld,” Mr Shannon said.
“But… I know that the work done to support me must be undertaken as economically and efficiently as possible.”
He added that he would be paying back the full amount.