The former Tory peer and Essex Council leader Lord Hanningfield has been found guilty of fiddling his expenses.
Paul White, 70, had denied six counts of false accounting relating to nearly £14,000 of parliamentary expenses.
The prosecution said he had claimed for overnight stays in London between March 2006 and April 2009, when he had actually returned home to Essex.
As he left court, White said he might appeal adding: "I'm devastated but I have no regrets. I did nothing wrong."
The jury at Chelmsford Crown Court found him guilty on all six counts and he will be sentenced in three weeks' time.
In a separate development, police have launched an investigation into an allegation Lord Hanningfield made fraudulent expenses claims of Essex County Council.
Members of the House of Lords were able to claim up to £174-a-night to stay in London when attending Parliament, if their main home was outside the city.
But peers were not asked for proof of how much had been spent and receipts were not required for car travel or rail tickets - if a single journey cost less than £25.
The trial heard White, of West Hanningfield, near Chelmsford in Essex, had made claims for journeys and overnight stays in London which he never made.
The prosecution said he had rented flats in London until 2006.
But on occasions between March 2006 and May 2009 when he made claims for overnight stays while attending Parliament, he usually made the 50-mile trip home, the trial heard.
Prosecutors also pointed to one occasion when he was on a plane bound for India at the time he claimed for accommodation, on others he was "at hotels outside London all paid for by someone else, mainly Essex County Council".
Peers do not get salaries but White earned £47,000 a year as leader of Essex County Council.
The peer argued he had subsidised his long years of public service out of his pocket, and said most other peers claimed the maximum under the allowance, which he saw as a "living-out-of-London allowance" rather than overnight subsistence.
When he was arrested, he told police: "It is an allowance scheme, not a reimbursement scheme. Quite honestly, people see it as a way of recouping what we spend."
White also said he had to return home to look after his Bernese mountain dog, Jefferson, and had incurred costs, including £20 a day for dog walking and housekeeping costs, which he could not claim.
He was convicted over claims totalling £13,379 for overnight stays in London, and for wrongfully claiming £382 in train fares and £147 mileage, by doubling the distance between his house and the train station. He has already paid back some of the claims.
White argued that many other peers claimed the maximum allowances and said he had "averaged out" claims to get back some of the money he had paid out for his parliamentary duties.
During his trial former Clerk of the Parliaments Michael Pownall said he believed peers regarded their expenses as "a reimbursement system" and most claimed the maximum allowed. But Mr Pownall said there was "no ambiguity" about the rule that you could only claim for overnight subsistence if you stayed in London.
White, who was an Essex councillor for 40 years and led the council from 2001 until he was charged in 2010, was made a life peer in 1998. He was a frontbench spokesman on business while the Conservatives were in opposition.
He was suspended from the Parliamentary Conservative party when he was charged.
He will remain a county councillor until his sentence has been determined but he has been removed from the Conservative group at county hall, an Essex County Council said in a statement.
But if he receives a jail term, he will not lose his peerage - unlike the House of Commons, the Lords does not have rules about expelling members who receive a custodial sentence. He could be suspended but only for a limited period.
Lord Hanningfield's was the last trial of the four former MPs and two peers charged over their expenses. As a result reporting restrictions have been lifted on the judge, Mr Justice Saunders', pre-trial hearings.
It has emerged that he strongly criticised Gordon Brown, David Cameron and Nick Clegg's remarks about the cases while legal proceedings had begun - and during a separate trial, that of former Tory peer Lord Taylor, he ordered Apprentice star Lord Sugar to remove comments he published about the case on the Twitter website.