Mr Leach's resignation had been widely expected following the company's embarrassing admission last week that it would make losses this year.
Yorkshire's problems have been caused by a shortfall in advertising revenues, which analysts estimate could be more than pounds 10m.
This was triggered when the group shifted responsibility for airtime sales from an in-house team to Laser, a subsidiary of the television group London Weekend Television.
LWT, which has a stake in Yorkshire together with the media group Pearson, was apparently very unhappy with the way the Yorkshire management had allowed the problem to arise. Mr Leach came under particular fire, partly because he has considerable experience in sales.
Contrary to earlier reports, however, Allan Hardy will remain the company's commercial director.
Mr Hardy, John Fairley, managing director of Yorkshire Television, and John Calvert, managing director of Tyne Tees Television, will report directly to Mr Thomas. A veteran of the industry at 69, Mr Thomas is viewed as a temporary appointment while the company identifies a permanent replacement.
This is unlikely until the new year, however. Peter Brooke, the National Heritage Secretary, is reviewing the rules that prevent the nine largest television companies buying each other, and a relaxation is widely expected that could determine Yorkshire's long-term future.
The option many expect is that the companies will be allowed a relatively free hand to merge, provided their resulting share of the advertising market does not exceed 25 per cent and with some protections for regional broadcasting in Scotland and Wales.
Both LWT and Granada are interested in buying Yorkshire, something that has sustained its share price, which closed down just 1p at 182p.
Most ITV shares have been inflated by the expectation of a widespread rationalisation that could reduce the present 15 companies to just three or four big groups.
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