Walker said: "After 14 years at Swanke, it got to the point where I would either have to stay there for the rest of my career or set up on my own.
"I wouldn't do it if I didn't have faith in finding work on the strength of my track record of large-scale urban projects."
Walker is the sole practitioner of David Walker Associates. With no fixed design commissions as yet, he is looking for premises near London's Oxford Street, and after that will advertise for staff.
"Hopefully, I will continue in some form of collaboration with Swanke," he added. "Though this has not yet been agreed."
The Merrill Lynch building won praise from English Heritage last year as an example of how to build big without building high.
It has also won awards from the Civic Trust and the City Heritage Society.