A spokesman for Sport England, which runs the Everyday Sport initiative, said: "Mook came out on top in our scoring system and demonstrated throughout the pitch process that it was best qualified to deliver the campaign's digital strategy."
Mook launched the programme last year with an online diary to record daily activities, with mobile and email prompts to encourage and motivate.
Famous faces including England and Manchester United star Rio Ferdinand, England rugby star Jonny Wilkinson, model Nell McAndrew, and singer Tony Hadley have backed the campaign, which is also supported by local authorities, hospitals, GP surgeries, businesses and sports clubs.
The announcement sees a continuation of the working relationship with Mook consultant Phil Jones, who worked with Sport England at direct marketing agency EHS Brann.
Earlier this month, Cheetham Bell JWT won the above-the-line work from incumbent agency Team Saatchi in a seven-way pitch.
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