William Hare will deliver the 5,300 tonnes of steel involved in the build while Focchi has been appointed as cladding contractor.
The announcement came as developer Great Western Developments Ltd agreed a £350m contract with Mace to complete the job.
The first phase of development has seen the completion of demolition and piling works by Erith.
Paul Flexney-Briscoe, Construction Director at Sellar, said: “We have worked extremely closely with Mace over the past year to frame a plan that is immediately actionable, on time and on brief, and we are happy to confirm today that it is to be delivered on budget.
“This is an important moment for the development, one that keeps us on track towards a successful build and completion, and marks the point that our vision for Paddington Square starts to become a tangible reality.”
Gareth Lewis, Mace’s Chief Executive for Construction, said: “The delivery of Paddington Square promises to create a new icon for the London skyline and create a new mixed-use neighbourhood in Paddington.
“The agreement of this contract is a major milestone that brings us closer to the completion of the scheme. Like the Shard before it, Mace is very proud to be working alongside Sellar to bring their ambitious vision for this new site to life.”
The mixed-use scheme will offer 360,000sq ft of office space across 18 floors, four tiers of retail, 1.35 acres of comprehensive public realm improvements and West London’s highest rooftop dining experience with views across the city’s skyline.
Paddington Square is set to be completed in spring 2022.