
Media regulator Ofcom is reported to have appointed headhunter Zygos Partnership to search for a replacement.
Johnson has been in the role for six years. He is the head of venture capital firm Risk Capital Partners and was a surprise choice when appointed in January 2004.
Adverts for the role are expected to be placed early next month. Past Channel 4 chairmen have included Sir Richard Attenborough, Sir Michael Bishop and Vanni Treves.