She said: "It was a very simple decision - I've been working for 17 years and have another 20 years ahead of me. I am not leaving because I'm bored of advertising or BMP. I'll see what's around when I'm ready to come back to work."
Her role will be assumed by the current joint deputy heads of planning, Lucy Jameson, a former Campaign Face to Watch, and Richard Butterworth.
Jameson joined BMP as a graduate trainee planner in 1993. She was appointed to the board in 1999 and was made the agency's joint deputy head of planning last summer. Butterworth also joined as a graduate trainee in 1992. He has been working on the agency's Volkswagen business since 1996 and was made the joint deputy head of planning alongside Jameson last year.
Jameson said: "Heads of planning at this agency have always been known for their thinking, smoking, mumbling and drinking. Between us, we should have those covered."
Johnson, who has worked on accounts including Anheuser-Busch, Barclaycard, Friskies Petcare Europe and Reuters, added: "Richard and Lucy are both outstanding planners. They know this place inside out."