Work on the first project in the £350m scheme on the western side of the town centre could start as early as next year.
With a development value of around £350m, the SG1 revamp plan, is one of the largest and most ambitious town centre regeneration schemes in the region.
The mixed-use regeneration scheme includes plans to create a series of new streets within Stevenage, as well as a redesigned, modern town square.
The development will also public services hub building that includes a new library and health facilities.
Residents and businesses will be invited to have their say through a comprehensive consultation exercise planned for this summer. Work is expected to take place over several phases with the first construction project commencing in 2020.
David Grover, Mace’s chief operating officer for development, said: “This agreement marks a major step forward for the transformation of Stevenage town centre.
“With a contract agreed we can now get started in earnest in delivering the regeneration of the town centre – I look forward to seeing our exciting plans come to life.”