The bank has suffered from IT project problems in the recent past, as well as facing calls from investors to beef up its board with more professional directors.
Thomas was previously senior vice president of HP Enterprise Services and was also executive vice president of technology firm EDS.
“I am delighted that Bill is joining the board,” said board chairman Richard Pym.
“The Co-operative Bank has a poor track record of successful IT investment and Bill’s experience from working in major global IT businesses will be of enormous help to the board in supervising our IT investment programme.”
Thomas’ other roles include working as a non-executive director at XChanging, GFI Software and Balfour Beatty.
The Co-op Bank’s troubles have come into focus this year after the Bank of England found a £1.5bn capital hole over the summer.
To fill that the Co-op Group is expected to lose control of the bank, giving more ownership to outside investors.
Those investors are expected to demand the board becomes tougher and more able to oversee the bank’s operations, covering areas like risk and capital management.
But it will also cover IT – the bank announced a further £148.4m write down of IT assets over the summer. It came after a project to build up the bank’s systems to cope with the purchase of 632 Lloyds branches was scrapped, as the Co-op pulled out of the deal.