Work will include a new roof, larger public areas and improvements to the surrounding infrastructure.
Anchor tenant Harvey Nichols will double the size of its current store under the plans from site owners Brockton Capital and partner Milligan Retail.
Brockton Capital partner Simon Samuels told the Birmingham Post: “We love what Birmingham City Council are doing to promote the city, including some very big infrastructure projects which are attracting additional investment into the city.
“And we love what that means for the Mailbox. The New Street redevelopment is going to be transformational – in fact the evidence is that it has been transformational already.”
Plans have been drawn up by architect Stanton Williams and work is set to begin in the summer and be completed by spring 2015.
Harvey Nichols will move to a new 45,000 sq ft signature store when the development is re-launched.