Danny Collinson, 29, became financial director of Hedon-based PGD Limited in August last year when he was one of seven people who bought it from owner Alex Hope.
What his six fellow directors did not know was that Collinson had been hooked on gambling since he began playing penny arcade machines in Withernsea at the age of ten.
He did not require any other signatories to access the company's bank account, which a judge said was "the equivalent of giving an alcoholic the key to the wine cellar".
Mr Tuniewicz asked Collinson to check online, but he said he could not access it at the time and agreed to accompany the other two men to a branch in Hull city centre.
Mr Hope and Mr Tuniewicz thought the account had been hacked, but Collinson knew the game was up and promptly disappeared, causing more than a week of anxiety for his family.
Dale Brook, prosecuting, told Hull Crown Court that as the men waited at the bank, "Mr Collinson indicated he needed to use the toilet, left the branch and never returned".
As Collinson caught a train to Edinburgh, Mr Tuniewicz was informed that more than £100,000 had been transferred out of the account that weekend.
In fact, in a little under two months, Collinson transferred a grand total of £488,234.42 from the account. The named recipients included Collinson, betting companies, HMRC, and firms working with PGD, but none other than Collinson had received a penny.
Before disposing of his mobile phone, Collinson sent to a text to relatives in which he "apologised for letting them down".
Police traced him to the Scottish capital, where he had been staying in a B&B. He was arrested on March 10 and had a rucksack full of cash and money in "various locations" amounting to more than £100,000.
A search of the home he shared with his parents in Hull Road, Withernsea, revealed legal "powders" and about £60,000.
He also had an Armani watch that appeared new.
Collinson, who was captain of Patrington Cricket Club and played football for Roos, admitted one count of theft.
But he had little to show for the fortune he had plundered and appeared to have blown most of it on gambling websites, the court heard.
Joanna Golding, defending, said: "From the age of ten he has been in the grips of the most devastating gambling addiction, starting from penny slot machines in Withernsea arcades and culminating in this monstrous theft from those with whom he worked."
Mrs Golding said Collinson fell into the addicted gambler's trap of placing ever bigger bets in the hope of recovering his losses.
"The more he lost the more he bet to try to recover the monies he was spending," she said.
"It became a never-ending spiral of desperation.
"This was a complete shock to his parents, from the little boy who used to go to the penny arcade to the master criminal who would end up in the situation he is in now.
"They can't comprehend how this happened."
She said: "There's a responsibility, too, for the people who took those bets, large sums, and they must have been aware that something was awry."
Sentencing Collinson to three years in jail, Judge Mark Bury told him: "I accept that as gamblers often do, they think that the answer to their prayers is in the next bet – it never is.
"I hope that either in custody or on licence, you get some help with your gambling addiction."