David Jamieson says the mayor, who is set to be elected in May 2017, will have a role that has already been 'reduced to the very minimum' and will not wield the power required to effectively run the West Midlands Combined Authority (WMCA).
The West Midlands Police and Crime Commissioner has also hit out at the proposed salary of the mayor, calling the figure of between £40,000 and £50,000 a year 'way below the expected level'.
Mr Jamieson's comments have been rejected by WMCA board members Councillor Roger Lawrence and Councillor Pete Lowe.
The WMCA includes the four Black Country councils and will see £36.5 million a year invested over 30 years, in housing, transport and infrastructure.
Mr Jamieson said: "What they are proposing does not match the ambitions that the Government has for the area in terms of the powers of the mayor.
"This should be a position of great authority, considering the mayor will preside over a large amount of economic development, but instead the board has shown political timidity. They should have made a much bolder decision.
"The role that has been proposed for our mayor has no comparison whatsoever to the powers of the Mayor of London. It is totally inadequate. We are left with a role that has been reduced to the very minimum."
On the mayor's proposed salary, which was revealed by WMCA chief executive Keith Ireland earlier this year, Mr Jamieson said: "It should be more like £140,000 rather than £40,000.
"The figure is way below the expected level. They appear to be looking for a chairman of a committee instead of a metro mayor."
The Mayor of London Sadiq Khan commands a salary of nearly £144,000, while the Greater Manchester mayor is expected to receive more than double the proposed salary for the West Midlands mayor.
Councillor Roger Lawrence, the WMCA's transport chief, said he 'completely rejects' Mr Jamieson's view.
He said the Mayor would play a vital role in the combined authority but said its success would be largely dependant on the abilities of the individual councils to deliver the scheme.
"The mayor and the cabinet will work as a team," he added. "For this to be a success we must act as one united voice."
The West Midlands mayor will be elected on May 4, 2017 and will serve a term of three years.