Andy Street's clear statement was met with rapturous applause from the crowd at St Bartholomew's Church in Penn on Wednesday evening.
He was in Wolverhampton for one of his regular Ask Andy events, where members of the public can come along and ask him questions on local issues.
Normally the events attract about 40 people but on Wednesday night, due to its prominent location on the door step of the Seven Cornfields, nearly 100 residents packed the pews at St Bartholomew's to question Mr Street about the "worrying" plans.
Retired Ros Norden has lived in the area near the green belt site for 40 years, and now lives on Brenton Road.
She said: "I'm very angered by it. I'm a person who uses Cornfields regularly and the original landowner said it would never be sold. People are very upset and emotional about this. It's a precious local green space.
"We want to fight for it, so it can be used for families here and now and families yet to come."
Seven Cornfields has been put forward for review as part of the Black Country Plan, which is looking at new areas for homes in the region to address a shortage.
Barratt Homes entered into an agreement with the landowner to develop proposals for up to 1,300 homes - but the move has sparked major backlash from local residents.
A petition to save the land from being built on has reached more than 5,000 signatures, which means it will now be discussed by Wolverhampton Council.
Andy Street said at the meeting: "I'm absolutely categoric about this.
"I will do everything I can to prevent this development. I'm not planning authority - that's down to Wolverhampton Council and South Staffordshire Council.
"So I cannot instruct officers but I can influence and make sure the right decision is taken."
Westbourne Road resident Rod Pickerill, age 67, said: "I've been walking over here 50 odd years, as a young lad and now with the dog. It's such a beautiful place. I can't believe they want to build houses on it, it's sacrilege in my opinion.
"There are plenty of brownfield sites in this area that can be developed on. There is no need to do it in this area."
After the meeting, the mayor added: "I would like to thank everyone who came to Penn to discuss the Seven Cornfields. The campaign to save this greenbelt land has been incredible, especially the speed in which it has gathered so much support.
“As I told residents, I will do whatever I can to prevent this proposed development. Greenbelt land across the West Midlands is precious and must be protected.
“The WMCA is not a planning authority so we cannot approve or reject proposals, but what we can do is spend the hundreds of millions of pounds awarded to us by central government to clean up derelict brownfield sites so they can be developed, as opposed to the greenbelt.
“I have always advocated a ‘brownfield first’ policy and will continue to work with local councils and developers to make sure this is implemented.”