Clowes, 56, sentenced to 10 years after the collapse of the investment company Barlow Clowes in 1992, had denied claiming pounds 1,800 in benefits when he was working.
It took the jury at Mold Crown Court in north Wales three hours to convict Clowes, now a computer programmer, living in Stockport, Greater Manchester.
Judge John Rogers QC said that the offence was aggravated by the fact that Clowes had been out of prison for just 10 months when he started making the false claims. The fraud was "sophisticated" and "deliberate", he added.
The judge told Clowes: "On December 3 you applied for job- seeker's allowance. You informed both orally and in writing the Benefits Agency that you were not working - that was untrue."
Judge Rogers told the court that had it not been for a change in the law, Clowes could have been ordered to serve some of the remainder of his previous sentence.
During the five-day trial, the jury heard that Clowes was accused of three specimen charges over a four-month period from December 1996.
Robin Spencer, for the prosecution, said Clowes had been working for a computer company in London, WP Software. However, he left that job and began claiming benefit while living with his wife and two children in a rented pounds 800-a-month four-bedroom home in Macclesfield, Cheshire.
Clowes then signed on and every fortnight he visited a Job Centre, renewing a declaration that he had not been working. But he was earning from a business trading under the name of PC51.
Mr Spencer said PC51 was set up by Mr Clowes' step-daughter and her boyfriend and he was paying his household bills from the business bank account. Clowes was being paid "through the back door", he added.
The court heard that Clowes was designing software for two firms at an old mill in Bollington, Cheshire.
Both firms were being operated by Christopher Jones and Philip Rich, the boyfriend of Nicola Haydock, Clowes' step-daughter.
The court was told that PC51 invoiced the accident claim firm Crash Direct and the window company Arctic Shields for new databases. Clowes then paid his rent, an electricity bill and the car tax on his wife's car from the PC51 account.
Clowes denied he had been working at the mill and said he had been studying the computer systems.
He said that the money from PC51 was a loan and that pounds 3,000 was paid into the business account by his parents.
Nigel Fieldhouse, for the defence, asked the judge to consider a sentence that would not "crush" his client.Reuse content