Johnson has been chairman of the broadcaster since January 2004, and will now be faced with a planned financial review of Channel 4 by media regulator Ofcom towards the end of the year, along with Channel 4's digital television and radio strategies.
David Currie, chairman of Ofcom, said: "Luke is an excellent chairman and ambassador for Channel 4, combining a commercial focus with a strong personal commitment to the principles of public service broadcasting."
Under Section 23 of the Broadcasting Act 1990, Ofcom is responsible for overseeing the appointment of the chairman of the Channel 4 Board.
Johnson's reappointment has been formally approved by Tessa Jowell, the secretary of state for culture, media and sport.
Johnson started his career in advertising with full-service agency BMP, subsequently joining Kleinwort Benson as a media analyst.
He became best known as an entrepreneur, floating the pizza chain Pizza Express in 1993.
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