Johnson and culture department ministers have announced a board of 12 members that will to sit for four years and be chosen by interview. Its establishment is to ensure locals have a bigger voice in the running of the parks, with significant involvement from the mayor.
But Murad Qureshi, chair of the assembly's environment committee, said the arrangements announced last week were different from those originally proposed. "To what extent are the Royal Parks the mayor's parks?" he asked.
GLA executive director Jeff Jacobs said the move represented a strong devolution package.
But while the mayor would appoint board members, final approval would be made by the Department for Culture, Media & Sport (DCMS), which could veto the mayor.
"There was never going to be a statutory change on the nature of what a Royal Park is. We are talking about the management arrangements. I would say the mayor has a clear ability to run the parks, but there are checks and balances. The parks remain part of the DCMS," Jacobs said.
Johnson will choose half of the members, including the chair. The remainder will include a member of the royal household and people chosen by local authorities and the Royal Parks Agency. The board will meet for the first time next month.