Her appointment highlights Hooper Galton's intention to place a greater emphasis on planning across the agency's client base and in its new-business activity.
Johnson joins after three years at Ogilvy & Mather, where she worked as the global planner on the Dove "real women" campaign.
Johnson's career spans more than 20 years and includes five years as an account planner at Bartle Bogle Hegarty and six years at the then BMP DDB.
Oliver Lewis Barclay, the strategic planner at Hooper Galton, said: "We've known Olivia since Martin Galton and I worked with her at BBH, and she has always been a brilliant thinker.
"She fits with the collaborative culture here perfectly and is exactly what we need to turbo-charge our strategic offering. We can't wait for her to get started."
Johnson added: "I like working in small, tight teams, and on campaigns that the public notice. Joining Hooper Galton will give me the chance to do both, on lots of different brands and across lots of new campaigns."
Hooper Galton's clients include Nestle, Emap, House of Fraser and New Covent Garden Soup Company.
In the 12 months to January 2005, Hooper Galton saw billings rise £1.2 million to £11.6 million, according to Nielsen Media Research.